Can we make mirrors which can give enlarged or diminished images?
Answer
Yes, we can make such mirrors. Spherical mirrors, which have curved reflecting surfaces, can give enlarged or diminished images. A concave mirror gives an enlarged image of the object when the object is placed close to it, while a convex mirror always gives a diminished image of the object.
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?
Answer
The side-view mirrors of vehicles are convex mirrors. A convex mirror always forms an erect and diminished (smaller) image of the traffic behind the vehicle. Because the vehicles appear smaller than their actual size, they seem to be farther away than they really are. Hence, the warning is written to remind the driver that the objects are actually closer than they appear in the mirror.
Why is there a curved line on some reading glasses?
Answer
The curved line on some reading glasses marks a lens portion with a different curvature/power, usually used for reading nearby text. This part helps in reading small print by making nearby letters appear clear and larger, like a magnifying glass.
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 40°. What is the angle made by the reflected ray with the mirror?
- 40°
- 50°
- 45°
- 60°
Answer
50°
Reason — The angle made by the incident ray with the normal (angle of incidence) is 40°. According to the law of reflection, the angle of reflection is also 40°.
Since the normal is at 90° to the mirror, the angle made by the reflected ray with the mirror = 90° − 40° = 50°.
Fig. 10.22 shows three different situations where a light ray falls on a mirror:
- The light ray falls along the normal.
- The mirror is tilted, but the light ray still falls along the normal to the tilted surface.
- The mirror is tilted, and the light ray falls at an angle of 20° from 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?
Answer
According to the law of reflection, the angle of reflection is always equal to the angle of incidence.
(i) The light ray falls along the normal, so the angle of incidence is 0°. Therefore, the angle of reflection is also 0°, and the ray is reflected back along the same path (along the normal).
(ii) The light ray falls along the normal to the tilted surface, so the angle of incidence is 0°. Therefore, the angle of reflection is also 0°, and the ray is reflected back along the same path.
(iii) The light ray falls at an angle of 20° from the normal, so the angle of incidence is 20°. Therefore, the angle of reflection is also 20°, measured on the other side of the normal.
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 | Mirror |
|---|---|
| (i) | Plane mirror |
| (ii) | Convex mirror |
| (iii) | Concave mirror |
Answer
| Image | Mirror |
|---|---|
| (i) | Convex mirror |
| (ii) | Concave mirror |
| (iii) | Plane mirror |
Reason — A convex mirror forms an erect and diminished (smaller) image, so image (i) is formed by a convex mirror. A concave mirror, when the object is placed close to it, forms an erect and enlarged (larger) image, so image (ii) is formed by a concave mirror. A plane mirror forms an erect image of the same size as the object, so image (iii) is formed by a plane mirror.
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 | Lens/glass type |
|---|---|
| (i) | Flat transparent glass piece |
| (ii) | Convex lens |
| (iii) | Concave lens |
Answer
| Image | Lens/glass type |
|---|---|
| (i) | Convex lens |
| (ii) | Concave lens |
| (iii) | Flat transparent glass piece |
Reason — A convex lens, when the object is placed close behind it, forms an erect and enlarged (larger) image, so image (i) is formed by a convex lens. A concave lens always forms an erect and diminished (smaller) image, so image (ii) is formed by a concave lens. A flat transparent glass piece does not change the size of the object, so image (iii) is seen through the flat transparent glass piece.
When the light is incident along the normal on the mirror, which of the following statements is true:
- Angle of incidence is 90°
- Angle of incidence is 0°
- Angle of reflection is 90°
- No reflection of light takes place in this case
Answer
Angle of incidence is 0°
Reason — The angle of incidence is measured between the incident ray and the normal. When the light is incident along the normal, the incident ray coincides with the normal, so the angle of incidence is 0°. By the law of reflection, the angle of reflection is also 0°, and the ray is reflected back along the same path.
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.
Answer
The left mirror is a convex mirror, the middle mirror is a plane mirror, and the right mirror is a concave mirror.
This can be identified by observing the size of the image of the graph sheet formed in each mirror.
In the left mirror, the image appears smaller than the actual graph sheet, which is a characteristic property of a convex mirror. In the middle mirror, the image is of the same size as the graph sheet, indicating that it is a plane mirror. In the right mirror, the image appears larger than the actual graph sheet, showing magnification, which is a property of a concave mirror when the object is placed close to it.
Therefore, from left to right, the mirrors are convex mirror, plane mirror, and concave mirror.
In a museum, a woman walks towards a large concave mirror (Fig. 10.26). She will see that:
- her erect image keeps decreasing in size.
- her inverted image keeps decreasing in size.
- her inverted image keeps increasing in size and eventually it becomes erect and magnified.
- her erect image keeps increasing in size.
Answer
her inverted image keeps increasing in size and eventually it becomes erect and magnified.
Reason — In a concave mirror, when the object is far from the mirror, the image is inverted. As the woman walks towards the mirror, the inverted image keeps increasing in size. When she comes very close to the mirror, the image becomes erect and magnified (enlarged).
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?
Answer
When the magnifying glass is held close to the text, the letters appear erect and enlarged, that is, bigger than their actual size. As the magnifying glass is moved away from the text, the letters first appear enlarged and then become inverted and start reducing in size. A magnifying glass is a convex lens.
Match the entries in Column I with those in Column II.
| Column I | Column II |
|---|---|
| (i) Concave mirror | (a) Spherical mirror with a reflecting surface that curves inwards. |
| (ii) Convex mirror | (b) It forms an image which is always erect and diminished in size. |
| (iii) Convex lens | (c) Object placed behind it may appear inverted at some distance. |
| (iv) Concave lens | (d) Object placed behind it always appears diminished in size. |
Answer
| Column I | Column II |
|---|---|
| (i) Concave mirror | (a) Spherical mirror with a reflecting surface that curves inwards. |
| (ii) Convex mirror | (b) It forms an image which is always erect and diminished in size. |
| (iii) Convex lens | (c) Object placed behind it may appear inverted at some distance. |
| (iv) Concave lens | (d) Object placed behind it always appears diminished in size. |
Reason — A concave mirror is a spherical mirror with a reflecting surface that curves inwards. A convex mirror always forms an erect and diminished image. An object placed behind a convex lens may appear inverted when it is at a large distance. An object placed behind a concave lens always appears diminished in size.
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:
- Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
- Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
- Assertion is correct but Reason is incorrect.
- Both Assertion and Reason are incorrect.
Answer
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Reason — Convex mirrors are used as side-view mirrors to observe the traffic behind us because, being curved outwards, they provide a much wider view of the road behind than a plane mirror. They form erect and diminished images, allowing a large area to be seen in a small mirror. Hence, both the Assertion and the Reason are correct, and the Reason correctly explains the Assertion.
In Fig. 10.27, note that O stands for object, M for mirror, and I for image.
Which of the following statements is true?
- Figure (a) indicates a plane mirror and Figure (b) indicates a concave mirror.
- Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.
- Figure (a) indicates a concave mirror and Figure (b) indicates a convex mirror.
- Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
Answer
Figure (a) indicates a convex mirror and Figure (b) indicates a concave mirror.
Reason — In Figure (a), the image (I) is smaller (diminished) than the object (O), which is a property of a convex mirror. In Figure (b), the image (I) is larger than the object (O), which is a property of a concave mirror.
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.
Answer
When the tumbler is filled halfway with water and the pencil is viewed through it, the part of the pencil seen through the water appears thicker, displaced, or bent compared to the part seen above the water level. Thus, the shape of the pencil appears changed.
This happens because the water-filled glass tumbler has a curved transparent surface and acts like a lens. It changes the path of light coming from the pencil before the light reaches our eyes. Due to this bending of light, the pencil appears thicker, displaced, or bent.