Light: Shadows and Reflections — Class 7 Science
Curiosity · Chapter 11
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7- Mathematics
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- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Social Science
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Luminous object
- An object that emits its own light, such as the Sun, stars, fire, or a firefly.
- Non-luminous object
- An object that does not emit its own light but may reflect it, such as the Moon or a mirror.
- Transparent, translucent, opaque
- Materials through which light passes almost completely, partially, or not at all, respectively.
- Shadow
- The dark patch formed when an opaque object blocks light travelling in a straight line; it needs a source, an object, and a screen.
- Reflection of light
- The change in the direction of light when it falls on a shiny surface or mirror.
- Lateral inversion
- The left-right reversal seen in the image formed by a plane mirror.
- Pinhole camera
- A device in which light from an object passes through a tiny hole to form an inverted image on a screen.
Extra questions & answers
What is the difference between luminous and non-luminous objects?
Luminous objects emit their own light, like the Sun and fireflies; non-luminous objects do not, like the Moon, which only reflects the Sun's light.
What do the matchbox and bent-pipe activities demonstrate?
They demonstrate that light travels in a straight line, since light is seen only when the holes or the pipe are aligned straight.
How do transparent, translucent, and opaque materials differ?
Light passes almost completely through transparent materials, partially through translucent materials, and not at all through opaque materials.
What three things are needed to observe a shadow?
A source of light, an opaque object to block the light, and a screen on which the shadow forms.
Does changing the colour of an object change the colour of its shadow?
No. Changing the colour of an opaque object does not change the colour of its shadow.
What is reflection of light?
Reflection is the change in the direction of light when it falls on a shiny surface or a mirror.
List the properties of an image formed by a plane mirror.
It is the same size as the object, erect, cannot be obtained on a screen, and is laterally inverted (left appears right).
Why is the word AMBULANCE written reversed on the front of the vehicle?
Because of lateral inversion, the reversed word reads correctly as AMBULANCE when seen in the rear-view mirror of a vehicle ahead.
What kind of image does a pinhole camera form?
A pinhole camera forms an inverted (upside-down) image of the object on its screen.
How does a periscope let you see over an obstacle?
A periscope uses two plane mirrors arranged in a Z-shaped box; reflection from the two mirrors lets you see objects that are not directly visible.
Let Us Enhance Our Learning
Which of the following are luminous objects? Mars, Moon, Pole Star, Sun, Venus, Mirror
Luminous objects are those that emit their own light. From the list, Pole Star and Sun are luminous because they are stars that produce their own light. Mars, Venus and Moon are non-luminous; they shine only by reflecting sunlight. Mirror is also non-luminous; it only reflects light falling on it.
Pinhole camera
Forms an inverted image
Opaque object
Blocks light completely
Transparent object
Light passes almost completely through it
Shadow
The dark region formed behind the object
Sahil, Rekha, Patrick, and Qasima are trying to observe the candle flame through the pipe as shown in Fig. 11.16. Who can see the flame?
Only Rekha can see the flame. Light travels in a straight line, so it can pass through a straight pipe but not through a pipe that is bent. In Fig. 11.16 Rekha is looking through the straight alignment, while the others are looking through a bent path where the light is blocked.
Look at the images shown in Fig. 11.17 and select the correct image showing the shadow formation of the boy.
The correct image is the one where the dark shadow is formed on the opposite side of the boy from the light source, behind the boy on the ground or screen. The shadow is erect with the same outline as the boy, but it is dark and not illuminated. Images that show the shadow on the same side as the light or with light inside the shadow are incorrect.
The shadow of a ball is formed on a wall by placing the ball in front of a fixed torch as shown in Fig. 11.18. In scenario (i) the ball is closer to the torch, while in scenario (ii) the ball is closer to the wall. Choose the most accurate representation of the shadows formed in both scenarios from the options provided (a and b).
Scenario (i) where the ball is closer to the torch produces a larger and more magnified shadow on the wall, because the object blocks a wider cone of light when it is near the source. Scenario (ii) where the ball is closer to the wall produces a smaller shadow that is closer to the actual size of the ball and sharper. Therefore the option showing a large shadow for (i) and a small shadow for (ii) is correct.
If the torch is close to the ball
The shadow would be larger
If the torch is far away
The shadow would be smaller
If the ball is removed from the set-up
A bright spot would appear on the screen
If two torches are present in the set-up on the left side of the ball
Two shadows would appear on the screen
Suppose you view the tree shown in Fig. 11.19 through a pinhole camera. Sketch the outline of the image of the tree formed in the pinhole camera.
The pinhole camera forms an inverted image. The tree will appear upside down on the tracing paper screen, with the top of the tree at the bottom and the bottom at the top. The colours of the tree are still seen but the image is dimmer and its size changes if you move the inner box forward or backward. The outline is the same shape as the tree but inverted vertically.
Write your name on a piece of paper and hold it in front of a plane mirror such that the paper is parallel to the mirror. Sketch the image. What difference do you notice? Explain the reason for the difference.
The image in the plane mirror appears laterally inverted. Your left appears as right and your right appears as left, so the letters of your name look reversed side-ways. For example, writing that is not symmetrical will look like mirror writing. This happens because a plane mirror causes lateral inversion and does not produce an upside-down inversion. The image is erect, same size as the object, but left-right swapped.
Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations: (i) At which of the given times is your shadow the shortest?
The shadow is shortest at 12 PM. At noon the Sun is almost overhead, so the light falls more vertically and the shadow is small and under your feet. In the morning and afternoon the shadow is longer.
Measure the length of your shadow at 9 AM, 12 PM, and 4 PM with the help of your friend. Write down your observations: (ii) Why do you think this happens?
This happens because the length of a shadow depends on the position of the Sun. When the Sun is low near the horizon at 9 AM and 4 PM, light comes at a slanting angle and makes a long shadow. When the Sun is high in the sky at 12 PM, the rays are more overhead, so the shadow becomes short. The Sun's apparent movement changes the angle of incidence and thus the shadow length.
On the basis of following statements, choose the correct option. Statement A: Image formed by a plane mirror is laterally inverted. Statement B: Images of alphabets T and O appear identical to themselves in a plane mirror. (i) Both statements are true (ii) Both statements are false (iii) Statement A is true, but statement B is false (iv) Statement A is false, but statement B is true
Option (i) Both statements are true is correct. Statement A is true because a plane mirror always shows lateral inversion. Statement B is also true because T and O are laterally symmetric, so after left-right reversal they look the same as the original.
Suppose you are given a tube of the shape shown in the Fig. 11.20 and two plane mirrors smaller than the diameter of the tube. Can this tube be used to make a periscope? If yes, mark where you will fix the plane mirrors.
Yes. Although the tube is bent, it is made of straight segments joined by two right-angle bends. Light travels in a straight line within each segment, so by fixing the two plane mirrors at the two bends (corners), each tilted at 45° and facing each other, light entering one end reflects at the first mirror, passes along the tube, reflects at the second mirror and emerges at the other end — so it works as a periscope.
We do not see the shadow on the ground of a bird flying high in the sky. However, the shadow is seen on the ground when the bird swoops near the ground. Think and explain why it is so.
When the bird flies very high, it is a small object very far from the ground. Because the Sun is a large, extended source, the bird blocks only a tiny fraction of its light; the full-shadow region (umbra) shrinks to almost nothing over such a large distance, and light from other parts of the Sun reaches the ground behind the bird. So no dark shadow forms on the ground. A clear shadow appears only when the bird is close to the ground/screen.
Exploratory Projects
Have you ever seen a firefly where you live? If no, ask your elders if fireflies were seen earlier in your region. If yes, find out the reasons for their not being seen anymore. Develop a story about it.
This is an observation and inquiry task. Students can note whether fireflies are seen in their area today. Many people report fewer fireflies than before. The chapter mentions reasons like light pollution, reduced forest and grass cover, and excessive tourism. Extra street lights at night disturb their light signals. A short story can describe how Keshav was excited to see fireflies in the village but learns they are disappearing and decides to reduce light pollution and protect habitats.
Repeat Activity 11.4, but this time cover the face of the torch with a coloured transparent paper and observe the colour of the shadow. Repeat this using transparent paper of different colours. Report your conclusions.
When the torch is covered with a coloured transparent sheet, the screen is lit with that colour, but the shadow formed by an opaque object remains dark, almost black, not coloured. Changing the colour of the object or the colour filter does not change the colour of the shadow, because a shadow is the region where light is blocked and no light reaches, so it remains dark regardless of the colour of the incident light.
A plane mirror forms only a single image of an object. But what will happen if two or more mirrors are kept at an angle with each other or parallel to each other? Find out by placing two mirrors as shown in Fig. 11.21.
When two plane mirrors are placed at an angle, multiple images are formed due to repeated reflection. The number of images depends on the angle between the mirrors; smaller angles give more images. When mirrors are parallel and facing each other, a large number of images are formed in a line due to infinite reflections. This principle is used in kaleidoscopes.
You are given a small piece of a plane mirror. Can this piece form an image of an object much larger than the mirror, like a large tree? Think and predict. Then carry out the activity.
Yes, even a small plane mirror can form an image of a large object. The image is complete but only a part of it is visible at a time depending on distance and angle. If you move farther from the mirror or change the viewing angle, you can see the whole tree. The size of the mirror does not limit the size of the object that can be imaged; it only limits the field of view seen at one position.