A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.
A screen is placed 90cm from an object. The image of the object on the screen is formed by a convex lens at two different locations separated by 20cm. Determine the focal length of the lens.
Solution and Explanation
Distance between the image (screen) and the object, D = 90cm
Distance between two locations of the convex lens, d = 20cm
The focal length of the lens = f
Focal length is related to d and D as:
\(f=\frac{D^2-d^2}{4D}\)
=\(\frac{(90)^2-(20)^2}{4\times 90}\)= \(\frac{770}{36}\)= 21.39cm
Therefore, the focal length of the convex lens is 21.39cm.
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Concepts Used:
Ray Optics and Optical Instruments
Optics, deals with the determination of behaviour and the properties of light, along with its interactions with the matter and also with the instruments that are used to detect it.
Ray optics is also known as the geometrical optics and it is a branch of science which describes light propagation.
Reflection is the change in direction of light at an interface in-between two different media so that the wave-front returns into a medium from which it was originated.
Speed of light is the rate at which the light travels in free space.
A phenomenal change in image formed when the light is passed from one medium to another which is called Refraction.
Total Internal Reflection is the reflection of light when the light ray enters into a rarer medium from a denser medium and the angle of incidence is higher than the critical angle of incidence then that light ray will be reflected back to the denser medium.
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