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Class 7 Science NCERT Notes – Chapter 11: Light, Shadows, and Reflections (PDF, MindMap, Q&A, Quizzes)
ByAKS
Chapter 11 (Science): Light, Shadows, and Reflections – CBSE Class 7 NCERT Science Detailed Study Notes.
1. Sources of Light
Light originates from various sources, which can be categorized as natural or artificial, and the objects themselves can be classified as luminous or non-luminous.
Luminous vs. Non-Luminous Objects
- Luminous Objects: These are objects that emit or give out their own light.
- Examples: The Sun, stars (like the Pole Star), lightning, natural fire, and certain animals like fireflies.
- Non-Luminous Objects: These objects do not produce their own light. They become visible because they reflect the light that falls on them from a luminous source.
- Example: The Moon is a non-luminous object. It shines in the night sky by reflecting the light emitted by the Sun. Other examples include Mars, Venus, and a mirror.
Natural vs. Artificial Sources
- Natural Sources: The Sun is the primary source of natural light on Earth. Other natural sources include stars, lightning, and bioluminescent organisms like fireflies.
- Artificial Sources: Humans have created various forms of lighting.
- Early Forms: The earliest artificial lighting was fire. Over time, fuels like animal fat, oil, wax, and gas were used to create light in devices like candles and lanterns.
- Modern Forms: With the invention of electricity, electric light sources became dominant. Light Emitting Diode (LED) lamps are modern, energy-efficient sources that consume less power, are brighter, and last longer than traditional lamps, making them economically and environmentally beneficial.
2. The Nature and Path of Light
A fundamental property of light is its path of travel.
Light Travels in a Straight Line
Observations and experiments confirm that light travels in a straight line.
- Observational Evidence: The beams from vehicle headlights appear to travel in straight lines.
- Experimental Evidence:
- Matchbox Experiment: If three matchboxes with holes in the center are aligned, a beam of light from a torch can pass through all three to form a spot on a screen. If any one of the matchboxes is moved, the holes are no longer in a straight line, and the light spot disappears from the screen.
- Pipe Experiment: A candle flame can be viewed through a straight, hollow pipe. If the pipe is bent, the flame is no longer visible, demonstrating that the light from the flame could not travel along the bent path.
- Laser Experiment: A low-power laser beam passed through a beaker of water (with a drop of milk to make the beam visible) shows a distinct straight path.
Note: The NCERT text mentions that light can sometimes bend around corners, a concept to be explored in higher grades.
3. How Light Interacts with Materials
Materials can be classified based on the amount of light they allow to pass through them.
- Transparent Materials: Allow light to pass through almost completely. Examples include glass and clear water.
- Translucent Materials: Allow light to pass through partially. An object viewed through a translucent material appears blurry. An example is tracing paper.
- Opaque Materials: Do not allow any light to pass through them. Examples include cardboard, thick cloth, and wood.
4. Shadow Formation
A shadow is a direct consequence of an opaque object blocking the straight path of light.
Conditions for Shadow Formation
To form a shadow, three components are essential:
- A source of light.
- An opaque object to block the light.
- A screen or surface behind the object where the shadow can be formed (e.g., a wall, the ground, a piece of cardboard).
If any of these components are removed (e.g., the torch is switched off, the object is removed, or the screen is removed), a shadow will not be formed.
Characteristics of Shadows
- Appearance: A shadow is a dark patch formed on a screen. The color of the opaque object does not change the color of the shadow; shadows are dark.
- Shape: The shape of a shadow can sometimes give information about the object, but it can also be misleading depending on the angle of the light and object.
- Material Interaction: Opaque objects form darker shadows. Translucent objects form lighter, fainter shadows. Some transparent objects can also create very faint shadows.
Factors Affecting Shadow Size
The size and sharpness of a shadow depend on the relative positions of the light source, the object, and the screen.
- Object close to the light source: The shadow becomes larger.
- Object close to the screen (far from the light source): The shadow becomes smaller.
5. Reflection of Light
When light hits a shiny surface, it changes direction.
- Definition of Reflection: The change in the direction of light by a mirror or another shiny surface is called reflection. Light continues to travel in straight lines before and after bouncing off the surface.
- Demonstration: Sunlight falling on a shiny steel plate or a plane mirror can be redirected to form a bright spot on a wall. Similarly, a thin beam of light from a torch passing through a comb slit can be seen reflecting off a plane mirror.
Plane Mirrors and Image Formation
A plane mirror is a mirror with a flat, reflective surface. What we see in a mirror is an image, which is a reflection of an object.
Characteristics of Images from a Plane Mirror
- Size: The image is the same size as the object.
- Orientation: The image is erect (upright). The top of the object appears as the top of the image.
- Screen Projection: The image cannot be obtained on a screen placed either in front of or behind the mirror.
- Distance: The image appears to be the same distance behind the mirror as the object is in front of it.
- Lateral Inversion: The image is laterally inverted, meaning there is a perceived left-right reversal. The object’s left side appears as the image’s right side, and vice versa. This is why the word “AMBULANCE” is written in reverse on the front of the vehicle, so it appears correctly in the rear-view mirrors of cars ahead.
6. Optical Instruments and Phenomena
Simple optical devices can be created to manipulate light and form images.
The Pinhole Camera
- Function: A simple device that forms an image of an object on a screen without a lens.
- Image Characteristics: The image formed by a pinhole camera is inverted (upside down).
- Construction: It can be made with two cardboard boxes, one sliding inside the other. The outer box has a small pinhole, and the inner box has a translucent screen (like tracing paper).
The Periscope
- Function: An instrument that allows observation of objects from a concealed position or around an obstacle.
- Construction: A Z-shaped box containing two plane mirrors fixed parallel to each other.
- Principle: Light from an object is reflected by the top mirror down to the bottom mirror, which then reflects it to the observer’s eye.
- Uses: Employed in submarines, tanks, and by soldiers in bunkers.
The Kaleidoscope
- Function: An optical toy that creates beautiful, symmetrical patterns.
- Construction: Typically made of three rectangular plane mirror strips joined to form a triangle inside a tube. Broken pieces of colored bangles or beads are placed at one end between a transparent sheet and a tracing paper screen.
- Principle: The multiple reflections of the colored pieces in the three mirrors create intricate and ever-changing patterns as the tube is rotated.
- Uses: Used by designers and artists to generate ideas for new patterns.
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Quiz and Q&A Section
Short-Answer Questions
Answer each question in 2-3 sentences.
- What is a luminous object? Provide two examples mentioned in the text.
- Explain why the Moon is considered a non-luminous object.
- According to the text, what are three factors contributing to the decrease in the firefly population?
- Describe the “pipe experiment” and what it demonstrates about the nature of light.
- How do LED lamps benefit society and the environment?
- Differentiate between transparent and opaque materials based on their interaction with light.
- What is a translucent material? What kind of shadow does it produce?
- What three components are necessary for a shadow to be formed?
- Explain how moving an object closer to a light source affects the size of its shadow.
- Does the color of an opaque object affect the color of its shadow? Explain why or why not.
- What is the definition of “reflection of light”?
- Describe the five main characteristics of an image formed by a plane mirror.
- What is lateral inversion? Provide a real-world example mentioned in the text.
- Can the image formed by a plane mirror be captured on a screen? Explain.
- How does the distance of an object from a plane mirror relate to the apparent distance of its image?
- What is the primary difference between the image formed by a pinhole camera and the image formed by a plane mirror in terms of orientation?
- Describe the basic construction of a simple periscope.
- What is the scientific principle that allows a periscope to work?
- Name two applications for periscopes mentioned in the text.
- How does a kaleidoscope create beautiful patterns?
- What is shadow puppetry, and what is its cultural significance in India?
- Name two regional styles of shadow puppetry in India mentioned in the text.
- What were early mirrors made of before the invention of glass mirrors?
- What is Aranmula Kannadi?
- How long does it take for light from the Sun to reach Earth?
Quiz: Multiple-Choice Questions (MCQ)
Choose the best answer for each question.
- Which of the following is a non-luminous object? a) Sun b) Lightning c) Venus d) Pole Star
- Light travels in a: a) Curved path b) Straight line c) Circular path d) Wavy path
- A material that allows light to pass through it partially is called: a) Opaque b) Transparent c) Luminous d) Translucent
- To observe a shadow, you need a light source, a screen, and a/an: a) Transparent object b) Opaque object c) Translucent object d) Mirror
- If you move an object closer to a screen (and away from the light source), its shadow will: a) Become larger b) Become smaller c) Stay the same size d) Disappear
- The image formed by a pinhole camera is always: a) Erect b) Inverted c) Laterally inverted d) The same size as the object
- The phenomenon where your left arm appears to be the right arm in a plane mirror’s reflection is called: a) Reflection b) Inversion c) Erection d) Lateral inversion
- An image formed by a plane mirror is: a) The same size as the object and erect. b) Smaller than the object and inverted. c) Larger than the object and erect. d) The same size as the object and inverted.
- Which device uses two plane mirrors in a Z-shaped box to see around obstacles? a) Kaleidoscope b) Pinhole Camera c) Periscope d) Telescope
- A kaleidoscope works on the principle of: a) Shadow formation b) Light traveling in a straight line c) Multiple reflections d) Light passing through a pinhole
- According to the text, the main source of natural light on Earth is: a) The Moon b) Lightning c) The Stars d) The Sun
- Changing the color of an opaque object will: a) Change the color of its shadow b) Make the shadow disappear c) Not change the color of its shadow d) Make the shadow lighter
- The image in a plane mirror appears to be located: a) In front of the mirror b) On the surface of the mirror c) The same distance behind the mirror as the object is in front d) Half the distance behind the mirror as the object is in front
- Shadow puppetry styles like ‘Togalu Gombeyaata’ and ‘Ravana Chhaya’ are part of the cultural heritage of: a) Europe b) India c) China d) Africa
- What material would you use for the screen of a pinhole camera? a) Thick cardboard b) A plane mirror c) Tracing paper d) Black paper
- Which of the following statements about an image in a plane mirror is FALSE? a) It is erect. b) It can be obtained on a screen. c) It is the same size as the object. d) It is laterally inverted.
- Light from the sun takes approximately how long to reach Earth? a) 8 seconds b) 20 minutes c) 8 minutes and 20 seconds d) 1 hour
- ‘Aranmula Kannadi’ from Kerala is a unique type of: a) Glass lens b) Shadow puppet c) Metal surface mirror d) Pinhole camera
- In the experiment to show that light travels in a straight line, what happens when one of the matchboxes with a hole is moved out of alignment? a) The light spot on the screen becomes bigger. b) The light bends to pass through the hole. c) The light spot can no longer be obtained on the screen. d) The light spot becomes brighter.
- A periscope is commonly used by: a) Artists and designers b) Astronomers c) Soldiers in bunkers d) City planners
Essay Questions
- Explain the difference between luminous and non-luminous objects using the Sun and Moon as primary examples. How do we see non-luminous objects?
- Describe two different experiments from the text that are used to demonstrate that light travels in a straight line. Explain the procedure and the conclusion for each.
- What are the three essential requirements for forming a shadow? Describe what happens to the shadow if each of these three requirements is removed one by one.
- Discuss how the characteristics of a shadow (size and shape) can be altered. Explain the relationship between the positions of the light source, the opaque object, and the screen.
- Compare and contrast the image formed by a plane mirror with the image formed by a pinhole camera. Discuss at least three distinct properties, including orientation and size.
- Explain the phenomenon of lateral inversion in detail. Use the example of looking at your reflection in a plane mirror and the text’s example of an ambulance to support the explanation.
- What is shadow puppetry? Describe its purpose and list three different regional styles of this art form found in India.
- Describe how to construct a simple kaleidoscope. Explain the scientific principle behind how it generates its unique and complex patterns.
- Explain the concepts of transparent, translucent, and opaque materials. How would the shadow cast by an object from each category differ?
- Describe how a simple periscope is constructed and how it works. Mention two practical applications of this device.
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Answer Keys
Short-Answer Questions Answer Key
- A luminous object is one that emits its own light. Examples mentioned in the text are the Sun and stars (like the Pole Star).
- The Moon is non-luminous because it does not emit its own light. It is visible only because it reflects the light from the Sun that falls on its surface.
- The text states that the number of fireflies is decreasing due to light pollution, reduced forest cover, and excessive tourism.
- The “pipe experiment” involves trying to see a candle flame through a hollow pipe. The flame is visible when the pipe is straight but becomes invisible when the pipe is bent, demonstrating that light travels in a straight line.
- LED lamps consume much less power, are brighter, and last longer than traditional lamps. This reduces electricity bills for society and is better for the environment.
- Transparent materials, like glass, allow light to pass through almost completely. Opaque materials, like cardboard, do not allow any light to pass through them.
- A translucent material is one that allows light to pass through partially, like tracing paper. It produces a lighter or fainter shadow compared to an opaque object.
- The three components necessary to form a shadow are a source of light, an opaque object to block the light, and a screen to cast the shadow upon.
- When an object is moved closer to a light source (and farther from the screen), its shadow becomes larger.
- No, the color of an opaque object does not affect the color of its shadow. Shadows are formed by the absence of light, so they appear as dark patches regardless of the object’s color.
- Reflection of light is the change in the direction of light when it falls on a mirror or a shiny surface.
- The image is (1) the same size as the object, (2) erect (upright), (3) cannot be obtained on a screen, (4) appears at the same distance behind the mirror as the object is in front, and (5) is laterally inverted.
- Lateral inversion is the perceived left-right reversal in an image formed by a plane mirror. An example is the word “AMBULANCE” written in reverse on vehicles so it can be read correctly in a driver’s rear-view mirror.
- No, the image formed by a plane mirror cannot be obtained on a screen. The text confirms this by describing an experiment where a screen placed behind or in front of the mirror fails to capture the image.
- The apparent distance of the image from a plane mirror is the same as the actual distance of the object from the mirror. If you stand closer to the mirror, your image also appears closer.
- The primary difference is that an image from a pinhole camera is inverted (upside down), while an image from a plane mirror is erect (upright).
- A simple periscope is made by placing two plane mirrors inside a Z-shaped box. The mirrors are placed parallel to each other.
- A periscope works by using two mirrors to reflect light. Light from an object hits the top mirror, reflects down to the bottom mirror, and then reflects again into the observer’s eye.
- The text mentions periscopes are used in submarines, tanks, and by soldiers in bunkers to see outside.
- A kaleidoscope uses three plane mirrors arranged in a triangle. The multiple reflections of colored objects placed at one end create beautiful, symmetrical patterns that change as the device is rotated.
- Shadow puppetry is an art form where flat cut-out figures (puppets) are manipulated between a light source and a screen to create moving shadows. It is a part of India’s cultural heritage used for entertainment and communicating important messages.
- Two regional styles mentioned are Charma Bahuli Natya in Maharashtra and Ravana Chhaya in Odisha. (Others include Tholu Bommalata, Togalu Gombeyaata, Tholpavakoothu, Bommalattam).
- Before glass mirrors, early mirrors were made by polishing materials like stone or metal.
- Aranmula Kannadi is a unique, traditional metal surface mirror that has been made for centuries in Kerala, India.
- Light emitted by the Sun takes about 8 minutes and 20 seconds to reach Earth.
Multiple-Choice Questions Answer Key
- c) Venus
- b) Straight line
- d) Translucent
- b) Opaque object
- b) Become smaller
- b) Inverted
- d) Lateral inversion
- a) The same size as the object and erect.
- c) Periscope
- c) Multiple reflections
- d) The Sun
- c) Not change the color of its shadow
- c) The same distance behind the mirror as the object is in front
- b) India
- c) Tracing paper
- b) It can be obtained on a screen.
- c) 8 minutes and 20 seconds
- c) Metal surface mirror
- c) The light spot can no longer be obtained on the screen.
- c) Soldiers in bunkers
Essay Questions Answer Key
- Luminous vs. Non-luminous Objects: Luminous objects are those that emit their own light, while non-luminous objects do not. The Sun is a prime example of a luminous object, as it generates and radiates its own light, which is the main source of natural light for Earth. The Moon, in contrast, is a non-luminous object; it does not produce any light itself. We see the Moon because its surface reflects the light from the Sun that falls upon it. In general, we see all non-luminous objects because they reflect light from a luminous source into our eyes.
- Experiments on Light’s Path:
- Experiment 1: The Matchbox Experiment. In this experiment, three matchboxes, each with a small hole in its center, are placed in a line. A torch is placed on one side, and a screen on the other. When the holes are perfectly aligned in a straight line, a spot of light from the torch appears on the screen. The conclusion is that because the light passes through all three aligned holes, it must be traveling in a straight line.
- Experiment 2: The Bent Pipe Experiment. This activity involves viewing a lighted candle through a long, hollow, flexible pipe. When the pipe is held straight, the candle flame is clearly visible. However, when the pipe is bent, the flame can no longer be seen. This demonstrates that light travels in a straight line and cannot bend to follow the curve of the pipe.
- Requirements for Shadow Formation: The three essential requirements for forming a shadow are: (1) a source of light, (2) an opaque object to block the light’s path, and (3) a screen or surface for the shadow to be cast upon. If the source of light is removed (e.g., the torch is switched off), there is no light to be blocked, so no shadow forms. If the opaque object is removed, the light travels uninterrupted to the screen, resulting in illumination but no shadow. If the screen is removed, the light is still blocked by the object, but there is no surface to capture and display the resulting dark patch, so a shadow cannot be observed.
- Altering Shadow Characteristics: The size and shape of a shadow are dependent on the relative positions of the light source, the object, and the screen. To alter the size, one can change the distance between these components. Moving the opaque object closer to the light source makes the shadow larger. Conversely, moving the object farther from the light source and closer to the screen makes the shadow smaller. The shape of the shadow can be altered by changing the orientation of the object relative to the light source; tilting the object will change the outline it presents to the light, thus changing the shape of the shadow cast on the screen.
- Pinhole Camera vs. Plane Mirror Image:
- Orientation: An image from a plane mirror is erect (upright), meaning the top of the object appears as the top of the image. In stark contrast, an image from a pinhole camera is inverted (upside down).
- Screen Projection: An image from a plane mirror cannot be formed on a screen. An image from a pinhole camera is, by its nature, formed on a screen (the tracing paper at the back of the camera).
- Other Properties: An image from a plane mirror is the same size as the object and is laterally inverted. The text does not specify the size of a pinhole camera image, as it depends on distances, but it is distinct from a mirror image in its inversion and formation on a screen.
- Lateral Inversion: Lateral inversion is the effect seen in images from plane mirrors where left and right are reversed. When you stand in front of a mirror and raise your left arm, your reflection appears to raise its right arm. Similarly, if you touch your right ear, the image appears to touch its left ear. The text provides the example of the word “AMBULANCE” being written as ” ̓” on the front of emergency vehicles. This is done so that drivers viewing the ambulance in their rear-view mirrors will see the word correctly spelled (“AMBULANCE”) due to lateral inversion, allowing them to quickly identify the vehicle and give way.
- Shadow Puppetry: Shadow puppetry, or shadow play, is an ancient art form where puppeteers use flat, cut-out figures called shadow puppets. These puppets are placed between a light source and a translucent screen, and their movements create life-like shadows that tell a story. This art is part of India’s cultural heritage and is used not only for entertainment but also to communicate important social and cultural messages to the community. Three regional styles mentioned in the text are Charma Bahuli Natya in Maharashtra, Tholu Bommalata of Andhra Pradesh, and Ravana Chhaya in Odisha.
- Kaleidoscope Construction and Principle: A simple kaleidoscope is constructed using three rectangular plane mirror strips of equal size, joined together to form a triangular prism. This assembly is fixed inside a circular tube. At one end of the tube, broken pieces of colored bangles or beads are placed on a transparent plastic sheet and covered with tracing paper. The scientific principle behind it is multiple reflections. Light from the colored pieces reflects back and forth between the three mirrors, creating multiple images of the pieces. These reflections of reflections form beautiful, complex, and symmetrical patterns that change every time the kaleidoscope is turned.
- Transparent, Translucent, and Opaque Materials:
- Transparent materials, like glass, allow light to pass through almost completely. They would cast either no shadow or a very faint one.
- Translucent materials, like tracing paper, allow light to pass through partially. They would cast a lighter, less dark shadow than an opaque object.
- Opaque materials, like wood or thick cloth, do not allow any light to pass through. They block light completely and therefore form a dark, well-defined shadow.
- Periscope Construction and Application: A periscope is made from a Z-shaped box with two plane mirrors placed inside it. The mirrors are set parallel to each other, each at a 45-degree angle to the vertical line of the tube. It works on the principle of reflection. Light from an object above the observer enters the top opening, hits the first mirror, is reflected down the tube, hits the second mirror, and is then reflected out horizontally into the observer’s eye. Two practical applications are its use in submarines to see above the water’s surface and by soldiers in bunkers or trenches to observe enemy positions without exposing themselves.
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Glossary of Key Terms
- Erect: An upright image, as formed by a plane mirror.
- Image: The reflection of an object, such as the face seen in a mirror.
- Inverted: An image that is upside down, as formed by a pinhole camera.
- Kaleidoscope: An optical instrument with mirrors that reflect images of colored glass pieces to create beautiful, changing patterns.
- Lateral Inversion: The perceived left-right reversal of an image in a plane mirror.
- Luminous Object: An object that emits (gives out) its own light.
- Non-luminous Object: An object that does not emit its own light and is seen by reflecting light from another source.
- Object: The actual item being reflected in a mirror or imaged by a camera.
- Opaque: A material that does not allow light to pass through it.
- Periscope: An optical instrument using mirrors to allow observation from a concealed or obstructed position.
- Pinhole Camera: A simple camera without a lens that forms an inverted image on a screen through a small aperture.
- Plane Mirror: A mirror with a flat (planar) reflective surface.
- Reflection of Light: The change in the direction of light by a mirror or other shiny surface.
- Screen: A surface on which a shadow or an image is formed.
- Shadow: A dark region formed on a surface when an object blocks the path of light.
- Translucent: A material that allows light to pass through it only partially.
- Transparent: A material that allows light to pass through it almost completely.