Apparatus:
Convex lens, three uprights, optical bench two parallax needles, a knitting needle and a meter rod.
Observations and Calculations:
Approximate focal length of the convex lens (f) =…………..m
Length of the knitting needle (L) =…………..cm
Observed distance between object and image needle (L1) =………….cm
Index correction object and image needle (L1-L2) =…………..cm
Observations and Calculations:
|
No. of obs. |
Position of | Distance between two needles ‘x’ | Position of the lens | d = L2-L1 | f = (x2-d2)/4x cm | |||
| Object needle “O” cm | Image needle ‘I’ cm | Observed cm | Corrected cm | 1st position L1 cm | 2nd position L2 cm | |||
| 1 2 3 |
||||||||
Procedure:
Determine the approximate focal length of convex lens by obtaining the distinct image on the paper screen or wall of a distant object. Place the convex lens in the lens upright. Mount the needle upright and lens upright on the bench. Measure the length of the knitting needle on the meter rod. Assign the object and image needle 0 and I respectively. Keep the distance between two needles must be more than four times the approximate focal length. Adjust 21/2 f on the image side and 11/2 f on the object side from the lens. Remove the parallax and note the position of uprights on their foot. This is the observed value of lens upright and needle uprights. Apply the index correction. Get the actual values of the observation.
Now change the position of The lens as L2 and again remove the parallax and note the new position of lens only. Find the displacement of the lens by L2-L1. For further readings increase the distance approx. 2 to 3 cm. Between two needles and follow the same procedure. Take at least three readings. Applying the formula and find the value of focal length of the convex lens.
Precautions:
1. Distance between the two needles should be more than four times the approximate focal length of the lens.
2. To remove parallax look from a large distance to avoid strain on the eyes.
3. For one observation the position of ‘0’ and ‘l’ should be kept fixed and only the lens should be displaced to the second position.
VIVA VOCE
Q.1. Define lens.
Ans. A piece of refracting medium bounded by one or two spherical surfaces is called lens.
Q.2. What is meant by principal focus of a convex lens?
Ans. The point at which the parallel beam of light is brought to focus after refraction through the lens is called as principal focus.
Q.3. What is meant by principal axis of convex lens?
Ans. The line passing through the optical center and its focus is called as principal axis.
Q.4. Define focal length of convex lens?
Ans. The distance from the center of lens to the principal focus is called as focal length.
Q.5. How will you differentiate between a convex lens and a concave lens?
Ans. Convex lens: Any lens which is thicker in the center and thinner at edges is called convex lens.
Concave lens: Any lens which is thinner in the center and thicker at edges is called convex lens.
Q.6. What is parallax?
Ans. The apparent shift of two objects or one object and its image by motion of eye is called parallax.
Q.7. What is meant by optical center?
Ans. A point in a lens through which a ray of light does not change its path is known as optical center.
Q.8. What is difference between real and virtual image?
Ans. Real image: The image that can be obtained on the screen and is always inverted is called as real image.
Virtual image: The image that cannot be obtained on the screen and is always erected called virtual image.
Q.9. What type of image is formed by convex lens and concave lens?
Ans. Convex lens forms a real image except when the object lies within focal length. Concave lens always form a virtual image.
Q.10. What is index correction?
Ans. It is the difference between the actual distance between two objects on uprights and the distance shown by the position of arrow marks on the foot of the two uprights i.e.,
Index correction = real distance – apparent distance
Q.11. What are main defects of lens?
Ans. There are two main defects of lens:
(1) Spherical aberration (ii) Chromatic aberration
Q.12. Why should the distance between two needles be more than 4f?
Ans. The distance between two needles be more than 4f, because for a convex lens the minimum distance between an object and its real image is 4.
Q.13. Define power of lens.
Ans. It is the reciprocal of focal length of the lens expressed in meters.
Q.14. What is the practical unit of power of lens?
Ans. The practical unit of power of lens is diopter. (D)
Q-15. Where will the image be formed by convex lens when the object is placed (a) at the principal focus (b) at 2F (c) between F and 2F?
Ans. (a) at infinity, (b) at 2F, (c) beyond 2F.
Q.16. Why is parallax not remove over the whole aperture of the lens?
Ans. This is due to spherical aberration
Q.17. Why is the focal length of convex lens taken +ve while that of concave lens taken –ve?
Ans. It is because in case of convex lens rays actually pass through the principal focus. While in case of concave lens rays do not actually pass through focus.
Q.18. What are conjugate foci of lens?
Ans. When an object is placed on principal focus so that its real image is formed by the lens, it is possible to put the object where the image is formed and obtain its image where the object was previously situated such interchangeable points are called conjugate foci.
Q.19. What type of image do concave lens and convex lens formed?
Ans. Convex lens always form a real image except when the object lies between its optical center and principal focus. While a concave lens always form a virtual image
Q.20. What would be the minimun distance between object and its real image formed by convex lens?Ans. It is exactly equals to 4 times of focal length of lens. (i.e. 41)
Q.21. What happens when light travel (a) from a denser to rarer medium (b) from a rarer to denser medium?
Ans. (a) it bends away from normal (b) It bend towards normal.
