Light: Mirrors and Lenses — Class 8 Science
Curiosity · Chapter 10Rising
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- 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
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Concave mirror
- A spherical mirror whose reflecting surface curves inward and which converges parallel light rays.
- Convex mirror
- A spherical mirror whose reflecting surface curves outward, always forming an erect, diminished image, and diverging light.
- Angle of incidence
- The angle between the incident ray and the normal at the point where light strikes a mirror.
- Laws of reflection
- The rules that the angle of incidence equals the angle of reflection and that the incident ray, normal, and reflected ray lie in one plane.
- Convex lens
- A lens thicker at the middle than at the edges that converges light; also called a converging lens.
- Concave lens
- A lens thicker at the edges than at the middle that diverges light and always forms an erect, diminished image.
Extra questions & answers
How do concave and convex mirrors differ in shape?
A concave mirror curves inward while a convex mirror curves outward.
What kind of image does a convex mirror always form?
An image that is always erect and smaller than the object, that is, diminished.
State the first law of reflection.
The angle of incidence is equal to the angle of reflection.
State the second law of reflection.
The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
Do the laws of reflection apply to spherical mirrors?
Yes, the laws of reflection are valid for all kinds of mirrors, including plane, concave, and convex.
What happens to parallel rays falling on a concave mirror?
They converge after reflection, which is why a concave mirror can focus sunlight to a bright spot.
Why are convex mirrors used as side-view mirrors on vehicles?
They always form erect, diminished images and give a wider view of the traffic behind.
How does a convex lens differ from a concave lens?
A convex lens is thicker in the middle and converges light, while a concave lens is thicker at the edges and diverges light.
What type of image does a concave lens always form?
An image that is always erect and diminished in size.
Name three devices that use lenses.
Spectacles, cameras, microscopes, and telescopes, and even the human eye has a convex lens.
Probe and ponder
Can we make mirrors which can give enlarged or diminished images?
Yes. Plane mirrors always give an image of the same size as the object. Spherical mirrors can give enlarged or diminished images. A concave mirror gives an enlarged erect image when the object is close to it and an inverted image which may be enlarged, same size or diminished as the object moves away. A convex mirror always gives an erect image which is diminished in size.
On side-view mirrors of vehicles, there is a warning that says “Objects in mirror are closer than they appear”. Why is this warning written there?
Side-view mirrors of vehicles are convex mirrors. A convex mirror forms an erect image that is smaller than the object and shows a wider area behind the vehicle. Because the image looks smaller and farther away than the real object, the driver may think the vehicle behind is farther than it actually is. The warning reminds the driver that the distance is underestimated.
Why is there a curved line on some reading glasses?
Some reading glasses are bifocal. The lower part has a different power for close reading and the upper part has a different power for distant viewing. The curved line separates the two lens regions which have different curvatures and powers. It marks the boundary between the two lenses joined together.
Share your questions
Examples of questions a student may ask are: Why does my image become inverted in a spoon when I move it away? Can a lens also burn paper like a concave mirror? Why does a convex mirror show more area than a plane mirror?
Keep the curiosity alive
1. A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is
The angle made with the normal is the angle of incidence
2. Fig. 10.22 shows three different situations where a light ray falls on a mirror: (i) The light ray falls along the normal. Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
Case (i): The incident ray is along the normal, so
2. Fig. 10.22 shows three different situations where a light ray falls on a mirror: (ii) The mirror is tilted, but the light ray still falls along the normal to the tilted surface. Draw the reflected ray in each case (Use a ruler and protractor for accurate drawing). What is the angle of reflection in each case?
Case (ii): Even though the mirror is tilted, the ray still meets it along the normal, so
2. Fig. 10.22 shows three different situations where a light ray falls on a mirror: (iii) The mirror is tilted, and the light ray falls at an angle of
Case (iii): The angle of incidence is
3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror. Image (i)
Image (i) which shows the cap of the same size as the object and erect corresponds to (i) Plane mirror. A plane mirror always forms an erect image of the same size.
3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror. Image (ii)
Image (ii) shows the cap enlarged and erect. Only a concave mirror can form an enlarged, erect image (object within its focal length), so image (ii) corresponds to (ii) Concave mirror. (A convex mirror can never enlarge.)
3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror. Image (iii)
Image (iii) shows the cap diminished and erect. A convex mirror always forms a diminished, erect image, so image (iii) corresponds to (iii) Convex mirror.
4. In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass. Image (i)
Image (i) shows the cap enlarged and erect. Only a convex lens can enlarge (object within its focal length), so image (i) corresponds to (i) Convex lens.
4. In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass. Image (ii)
Image (ii) shows the cap diminished and erect. A concave lens always forms a diminished, erect image, so image (ii) corresponds to (ii) Concave lens.
4. In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece — all at the same distance. Match each image with the correct type of lens or glass. Image (iii)
Image (iii) shows the cap the same size as the object, seen through the rectangular slab. A flat transparent glass piece does not enlarge or diminish, so image (iii) corresponds to (iii) Flat transparent glass piece.
5. When the light is incident along the normal on the mirror, which of the following statements is true: (i) Angle of incidence is
The correct statement is (ii) Angle of incidence is
6. Three mirrors—plane, concave and convex are placed in Fig. 10.25. On the basis of the images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.
The mirror that shows the graph squares of the same size as the object is the plane mirror. The mirror that shows magnified squares is the concave mirror when the object is close. The mirror that shows diminished squares is the convex mirror. In the figure, the leftmost mirror with diminished image is convex, the middle with same size is plane, and the rightmost with enlarged image is concave, with left-right inversion seen in all three.
7. In a museum, a woman walks towards a large concave mirror (Fig. 10.26). She will see that: (i) her erect image keeps decreasing in size. (ii) her inverted image keeps decreasing in size. (iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified. (iv) her erect image keeps increasing in size.
The correct option is (iii): her inverted image keeps increasing in size and eventually becomes erect and magnified. When she is far from the large concave mirror, the image is real and inverted; as she approaches, this inverted image grows, and once she is within the focal length the image flips to erect and magnified (as the figure shows the inverted image at a distance). Option (iv) is wrong because far away the image is inverted, not erect.
8. Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
A magnifying glass is a convex lens. When it is held close to the text at a small distance, the text appears erect and enlarged. As it is moved away, the enlarged image grows larger and then at a certain distance it becomes inverted. On further increasing the distance it appears inverted and then its size decreases. The convex lens converges light and can magnify when the object is close.
9. Match the entries in Column I with those in Column II. Column I (i) Concave mirror
(a) Spherical mirror with a reflecting surface that curves inwards.
9. Match the entries in Column I with those in Column II. Column I (ii) Convex mirror
(b) It forms an image which is always erect and diminished in size.
9. Match the entries in Column I with those in Column II. Column I (iii) Convex lens
(c) Object placed behind it may appear inverted at some distance.
9. Match the entries in Column I with those in Column II. Column I (iv) Concave lens
(d) Object placed behind it always appears diminished in size.
10. The following question is based on Assertion/Reason. Assertion: Convex mirrors are preferred for observing the traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors. Choose the correct option: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion. (ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion. (iii) Assertion is correct but Reason is incorrect. (iv) Both Assertion and Reason are incorrect.
Option (i) is correct. Convex mirrors are preferred as side-view and road safety mirrors because they are curved outward and give a wider field of view than plane mirrors, and they always form an erect diminished image, allowing the driver to see more traffic behind. So both Assertion and Reason are correct and Reason correctly explains Assertion.
11. In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true? (i) Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror. (ii) Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror. (iii) Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror. (iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
The correct statement is (iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror. In Fig. (a) the image distance and size appear equal to the object distance, characteristic of a plane mirror. In Fig. (b) the image is smaller and closer to the mirror than the object, characteristic of a convex mirror which always forms a diminished image.
12. Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.
When the tumbler is empty the pencil appears normal. When water is filled halfway, the part of the pencil seen through the water appears thicker, shifted or broken compared to the part above water. This is because the water in the cylindrical tumbler acts as a convex lens. Light rays from the pencil refract on passing through the curved water-glass surface and converge, so the image appears magnified and displaced. The curved surface causes refraction similar to a lens, changing the apparent size and position of the pencil.
Discover, design, and debate
Visit a nearby hospital or the clinic of an ENT specialist, or a dentist, with your teacher or parents. Request the doctor to show you the mirrors used for examining ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments.
Doctors use concave mirrors for examination. A concave mirror converges light and when the object like teeth or the ear canal is close to it, it gives an enlarged erect image. The ENT head mirror and the dentist's mouth mirror are concave mirrors that help to see a magnified and well-illuminated view of the small cavities.
Harnessing sunlight is key to solving future energy challenges. In devices like solar cookers (Fig. 10.29), mirrors are used to converge sunlight and generate heat. In India, such designs are used in villages, thus saving electricity and reducing fossil fuel use. Think of a design for a solar cooker for your school or home and prepare a detailed proposal for it including the budget required.
A simple solar cooker can be made with a cardboard box lined with aluminium foil as a concave reflector to concentrate sunlight onto a black-coated vessel. The proposal would include: aim to cook rice or boil water using solar energy, materials like cardboard, foil, glass lid, black pot, approximate cost under
Use online tools or animation to do virtual experiments with spherical mirrors and lenses. Move objects in the simulation and observe how the image changes.
In a virtual simulation, moving the object towards and away from a concave mirror shows the image changing from erect enlarged when close to inverted enlarged, then same size and then diminished as distance increases. For a convex mirror the image stays erect and diminished throughout, only its relative size changes slightly. For a convex lens the image is erect enlarged when close and becomes inverted on moving away, while a concave lens always gives an erect diminished image. This confirms the observations of Activities 10.3 and 10.9.