What do you understand by a convex lens and a concave lens? On an optical bench

Question

What do you understand by a convex lens and a concave lens? On an optical bench an object is in front of a convex lens. There is also a screen placed on the optical bench which shows a sharp image of the object. Define a. Object distance b. Image distance A 1 cm tall object is placed on the optical axis of a thin lens, 30 cm away from the lens which has a focal length of +20 cm and the image is observed on the screen. a. What is the object distance? b. How far the image will be from the lens c. What is the size of the image? d. Is it a virtual or real image 4. Answer question 3 above for a lens with focal length -20cm Consider the following diagram, in which a single lens is used to form an image of an arrow-shaped object Which of the following statements is true? a. If the object is moved to the right, the image will also move to the right b. If the object is moved to the right, the image will move to the left. c. If the object is moved to the right, the image will not move since the focal length of s. the lens has not changed

Explanation / Answer

Object distance is the distance between the object and the lens.
Image distance is the distance between the screen where the sharp image of the object is formed and the lens.
A.
a. The object is placed 30 cm from the lens. So the object distance is 30 cm
Object distance u = 30 cm
b) The relation between the object distance, image distance and the focal length is given as
1/ u + 1/ v = 1 / f
Where v is the image distance and f is the focal length.
The focal length of the convex lens is given as f = 20 cm
1/ v = 1 / f - 1/ u
v = f u / ( u - f)
v = 30 x 20 / (30 - 20)
v = 60 cm
This is the image distance.
c) The magnification of the image is related to the object and the image distance as
M = - v / u
The magnification is the ratio of image distance to the object distance
M = hi / ho = - v / u
hi = - ho (v /u)
hi = - 1 x (60 / 30)
hi = - 2 cm
The image is 2 cm tall.
d) The negative sign of the image height shows that the image is virtual.
A virtual image is formed.

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