Advertisements
Advertisements
Question
At what distance from a concave lens of focal length 20 cm, should a 6 cm tall object be placed so that it forms an image at 15 cm from the lens ? Also determine the size of the image formed.
Solution
Focal length of concave lens, f = −20 cm
Image distance, v = −15 cm (Negative sign is taken because of sign convention.)
Using the lens formula, we get:
`1/f=1/v-1/u`
`rArr1/(-20)=1/(-15)-1/u`
`rArr1/u=1/(-15)+1/20`
`rArr1/u=(-4+3)/60`
`rArru=-60" cm"`
The distance of the object from the mirror is 60 cm.
`h_i`is the size of the image formed.
`h_o`m=vu=hiho
We know that the image formed by the concave lens is virtual and erect.
The magnification is given as:
`m=v/u=h_i/h_o`
`(-15)/(-60)=h_i/6`
`rArrh_i=1.5" cm"`
Hence, the size of the image is 1.5 cm.
APPEARS IN
RELATED QUESTIONS
The image formed by a spherical mirror is real, inverted and is of magnification -2. If the image is at a distance of 30 cm from the mirror, where is the object placed? Find the focal length of the mirror. List two characteristics of the image formed if the object is moved 10 cm towards the mirror.
The image of a candle flame placed at a distance of 30 cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 60 cm from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 3 cm, find the height of its image.
At which position will you keep an object in front of a convex lens so as to get a real image of the same size as the object? Draw a figure.
Give scientific reason.
Simple microscope is used for watch repairs.
The lens of the eye is flattened when looking at nearby objects.
An object is placed vertically at a distance of 20 cm from a convex lens. If the height of the object is 5 cm and the focal length of the lens is 10 cm, what will be the position, size and nature of the image? How much bigger as compared to the object?
The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between lens and image is 80 cm, at what distance should the candle be placed from the lens? What is the nature of the image at a distance of 80 cm and the lens?
A lens of focal length 5 cm is being used by Debashree in the laboratory as a magnifying glass. Her least distance of distinct vision is 25 cm.
- What is the magnification obtained by using the glass?
- She keeps a book at a distance 10 cm from her eyes and tries to read. She is unable to read. What is the reason for this?
The magnification by a lens is -3. Name the lens and state how are u and v related?
What information about the nature of image is erect or inverted, do you get from the sign of magnification + or -?
