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Question :-
Study the given diagrams and select the correct statement from the following.
Options :-
1) Device X is a concave mirror and device Y is a convex lens, whose focal length are 20cm and 25cm respectively.
2)Device X is a convex lens and device Y is a concave mirror, whose focal length are 10cm and 25cm respectively.
3) Device X is a concave lens and device Y is a convex mirror , whose focal length are 20cm and 25cm respectively.
4) Device X is a convex lens and device Y is a concave mirror , whose focal length are 20cm and 25cm respectively.
[Kindly give reason also ]
Class - 10th
Chapter - Light, Reflection and Refraction.
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Answers
..QUESTION..
Study the given diagrams and select the correct statement from the following .
OPTIONS PROVIDED :
- Device X is a concave mirror and device Y is a convex lens, whose focal length are 20cm and 25cm respectively.
- Device X is a convex lens and device Y is a concave mirror, whose focal length are 10cm and 25cm respectively.
- Device X is a concave lens and device Y is a convex mirror , whose focal length are 20cm and 25cm respectively.
- Device X is a convex lens and device Y is a concave mirror , whose focal length are 20cm and 25cm respectively.
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..ANSWER..
- ➪ In figure 1 , we are able to understand that the rays which are passing through the Device(x) are concentrated at a distinct points on the screen . Therefore device(x) is (convex lens).
- example : If sunlight is allowed to pàss through a magnifying glass . It's rays are concentrated at one point making a tiny spot with all heat energy .
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- ➪ In figure 2 , we are given that the rays falls on the screen on a concentrated nature , reflecting from device(y) . The scattering light that reflects in small amount meeting the principle focus indicates that device(y) is (Concave mirror).
- example : A concave structure of mirror or other shiny object is placed inside the headlight of cars to focus the light source to one point .
★ Since the diagrams also provides the length i.e 20 cm and 25 cm respectively .
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•°• Correct option :
4 ) Device X is a convex lens and device Y is a concave mirror , whose focal length are 20cm and 25cm respectively .
ㅤㅤㅤ꧁ ʙʀᴀɪɴʟʏ×ᴋɪᴋɪ ꧂
Understanding the figure 1:
In the given figure, we can see that the rays are coming from infinity, passing through the surface and converging at a point.
Since rays are passing through the surface, it is a lens. Now the question arises that what type of lens is it? Answer is simple, since light rays are converging at a point by a lens, it must be converging lens or convex lens. Also we know that whenever light rays coming from infinity meets at focus, and the distance between focus and pole is called focal length, so it's focal length is 20cm.
From figure 1, we get:
• The screen x is convex lens.
• Focal length of the given convex lens is 20cm.
Understanding the figure 2:
In the given figure, we can see that the light rays are not passing through the surface but are bouncing back along different directions. We know that a lens can't reflect a light ray. A light ray can only be refract by a lens. Hence it is a mirror. Now again after observing, we can see that all light rays are converging at a point. Since converging mirror is also called concave mirror, it is a concave mirror. Light rays after coming from infinity converge at focus. As are aware that distance between focus and pole is called focal length, the focal length of Y is 25cm.
From figure 2, we get:
• The surface Y is concave mirror.
• Focal length of Y is 25cm.
Note that image formed in both the cases is on screen and we know that only real image can be formed on screen.
From our above observations:
Device X is convex lens and device Y is concave mirror, whose focal length are 2cm and 25cm respectively.
Hence option (4) is correct.
Addition Information:
Position and nature of images formed by concave mirror: