Refraction light:
Bending of light as it passes through one transparent medium to another medium is called refraction of light. For refraction of light it requires one rarer medium and another denser.
Lens:
A lens is a portion of transparent medium bounded by two refracting surfaces at least one of which is spherical.
Convex Lens:
A lens which is thicker at center than edges is called convex lens. It is also called converging lens. Power of convex lens is taken as positive.
Concave Lens:
A lens thinner at center than edges is called concave lens. It is also called diverging lens. Its power is taken as negative.
Position and nature of image formed by convex lens.
i. When object is at infinity, image is formed at focus, Nature of image: Real, inverted and highly diminished.
ii. When object is placed beyond 2F. Image is formed between F and 2F. Nature of image: Real inverted and diminished.
iii. When object is placed at 2F, image is formed at 2f. Nature of image: Real, inverted and same size of object.
- When an object is placed between F and 2F, image is formed beyond 2f. Nature of image: Real, inverted and magnified.
- When an object is placed at F, image is formed at infinity. Nature of image: Real, inverted and highly magnified.
- When an object is placed between F and optical center ‘O’ image is formed behind the object. Nature of image: Virtual, erect and magnified.
Position and nature of image formed by concave lens. Wherever be the position of object in concave mirror, image is always formed between F and O. Nature of image: Virtual, erect and diminished.
| Real Image | Virtual Image |
| It is formed at the point where refracted rays meet. | It is formed at the point where refracted rays appear to meet |
| It is always inverted | It is always erect. |
| Usually it is formed an another side of lens. | It is always formed on the same side of object. |
| It can be obtained on screen. | It cannot be obtained on screen. |
Uses of Convex Lens:
i) It is used to make, microscope, telescope, camera etc.
ii) It is used to correct long-sightedness defect of eye.
Uses of concave Lens:
i) It is used to obtain clear image with convex lens in camera, telescope, microscope etc.
ii. It is used to correct short-sightedness defect of eye.
| Convex Lens | Concave Lens |
| 1. It is thick at middle and thin at edges. | 1. It is thin at middle and thick at edges. |
| 2. It is focus is real. | 2. Its focus is virtual. |
| 3. It forms both real and virtual images. | 3. It forms virtual image. |
Magnification:
Ratio of size of image formed to the object is called magnification. It is denoted by ‘M’.
Magnification (M)=
Also,
Magnification (M)=
\[M=]
Power of Lens:
Capacity of the lens to converge or diverge the parallel rays of light is called power of lens. It is calculated as. P= where P= power of lens and F= Focal length in meter. Power of lens is measured in dioptre (D) unit.
Optical instrument: The instrument which forms image of an object is called optical instrument. Optical instrument are of two type;
i. Forming real image, eg: camera, eye, projector
ii. Forming virtual image, eg: microscope, telescope etc.
iii. Light entering in the camera is controlled by the shutter and aperture.
Eye
Power of accommodation:
The ability of eye to change the focal length of eye lens so as to obtain image always on retina is called power accommodation. Ciliary muscle maintains this process by contraction and relaxation.
Defect of vision:
i) Short sightedness
ii) Long sightedness
Short sightedness: The defect of vision in which person cannot see the distant objects clearly is called short sightedness. In this case image is formed in front of retina.
Causes of Short sightedness:
- Elongation of eye ball
- Decreasing focal length of eye lens.
Remedy:
Using concave lens of suitable power.
Long sightedness:
The defect of vision in which person can see distant object clearly but not object at near point is called long sightedness. In this case image is formed behind retina.
Causes of Long sightedness:
- Shortening of eye ball
- Increasing focal length of eye lens.
Remedy:
Using convex lens of suitable power.
Compound Microscope:
i. It uses two convex lens, among which lens closer to object is called objective lens and close to eye is called eye piece.
ii. It makes very small object bigger.
iii. It forms virtual, erect and highly magnified image.
Working:
When an object is placed beyond focus of objective real, inverted and magnified image is formed. Eyepiece has been adjusted in such a way that this image forms in between focus and optical center of eye piece. So that highly magnified virtual image is formed.
Telescope:
The optical instrument used to see distant object clearly is called telescope.
- It has two convex lens , objective and eye piece.
- Objective lens is bigger than eye piece for purpose of collecting large light from object.
- Focal length of objective is greater than eye piece.
REASONABLE FACTS
1. Convex lens is called converging lens.
Ans- As the convex lens converges the parallel rays of light at a point after refraction, it is called as converging lens.
2. Convex lens is used in making magnifying lens.
Ans- As the convex lens forms virtual and magnified image of object when placed between focus and optical centre, it can be used in making magnifying lens.
3. Concave lens is called diverging lens.
Ans- As the concave lens diverges the parallel rays of light after refraction; it is called as diverging lens.
4. Focusing is done in camera, microscope and telescope.
Ans- Focusing is done to produce a sharp and clear image in camera, microscope, and telescope and human eyes.
5. We cannot see the things clearly around us when we enter in the dark room from bright sunshine.
Ans- Due to the change in size of pupil (becomes small) or our eyes in the bright sunshine, so we cannot see the things clearly around us when we enter in the dark room from bright sunshine.
6. The normal eyes cannot see the thing placed at a distance of less than 25cm.
And- The normal eyes cannot see the things placed at a distance of less than 25cm because the ciliary muscle falls to make the eye lens thick to decrease its focal length.
7. A lens refracts light.
Ans- A lens refract light because of change in speed of light while passing through the lens as lens is a denser medium.
8. Objective lens has length in telescope.
Ans- Objective lens has a very greater focal length in telescope to form large image.
9. Objective lens has larger diameter (aperture) in telescope than compound microscope but eye-piece has small diameter.
Ans- To collect more light from distant objects resulting in the formation of bright image of them, the objective lens is made wider than that of compound microscope and to make clear visibility of the objects as the whole light enters the eye from small aperture, eye-piece has small diameter (aperture).
10. Focal length of convex lens is taken as positive.
Ans- Focal length of convex lens is taken as positive (by sign convection) since convex lens has real focus and hence real focal length.
11. Focal length of concave lens is taken as negative.
Ans- Focal length of concave lens is taken as negative (by sign convection) since concave lens doesn’t have real focus but have virtual focal length.
12. Concave lens cannot be used to view microscopic objects.
Ans- Concave lens always forms virtual and diminished image so that it cannot be used to view microscopic objects.
13. A light ray bends away from principal axis when it passes through a concave lens.
Ans- When the light ray passes through a prism, it bends towards the base of prosm but in case of concave lens the base of prism faces away from the principal axis. So, a light ray bends away from principal axis when it passes through a concave lens.
14. A light ray bands towards the principal axis when it passes through a convex lens.
Ans- When the light ray passes through a prism, it bends towards the base of prism but in case of convex lens the base of prism faces towards the principal axis. So, a light ray bends towards the principal axis when it passes through a concave lens.
15. Magnifying glass of smaller focal length is usually preferred.
Ans- Magnifying glass has more magnifying power when focal length is small, so magnifying glass of smaller focal length is usually used.
16. The objective lens of a compound microscope has a small diameter.
Ans- To collect on only adequate amount of light from the objects resulting in the formation of clear visibility of them, the objective lens of a compound microscope has a small diameter (aperture).
17. Concave lens cannot be used in making hand lens.
OR
Concave lens cannot be used to view microscopic objects.
Ans-Concave lens always forms virtual and diminished image so that it cannot be used in making hand lens nor in viewing microscopic objects.
18. Convex can be used in making hand lens.
Ans- Convex lens forms the virtual and magnified image so that it can be used in making hand lens.
19. Combination of lens is done.
Ans- To increase the sharpness of the image formed, the combination of lens is done.
20. A paper gets burnt when the sunlight is focused on it.
Ans- Due to the concentration of infra-radiation on small area of paper, the heat energy increases so that a paper gets burnt.
21. The distance between objective lens and the eye lens in a compound microscope doesn’t have to be changed, but it has to be changed in a telescope.
Ans- The image of an object made by objective lens in compound microscope lies always in same point, so in a compound microscope it isn’t necessary to change the distance between objective lens and the eye lens.
But, due to the lack of fixed position of the object, it is difficult to adjust the distance between objective lens and the eye lens. So, it is necessary to adjust distance between objective lens and the eye lens by changing so as to place the image on the focus (to make focal point of the eye lens lie on the image formed by objective lens) so that the object can be clearly seen.
22. Concave lens is used in the correction of myopia (short-sightedness).
Ans- Concave lens diverges all the light rays and helps the lens to produce the image on the retina so it is used in the correction of myopia(long-sightedness).
23. Convex lens is used in the correction of hyper-metropia (long-sightedness)
Ans-Convex lens converges all the light rays and shortens the effective focal length of the eye lens helps the lens to produce the image on the retina so it is used in the correction of hyper-metropia (long-sightednes).
24. The power of the convex lens is positive.
Ans- As the focal length of a convex lens is taken as positive due to its real focus, so its power is also positive.
25. The power of the concave lens is negative.
Ans- As the focal length of a concave lens is taken as negative due to its virtual focus, so its power is also negative.
26. Objective lens of a compound microscope has shorter focal length than the eye piece.
Ans- Objective lens of a compound microscope has shorter focal length than the eye piece to increase magnification and to control unnecessary light to pass in the microscope.
27. Objective lens of telescope has longer focal length than the eye piece.
Ans- Objective lens of telescope has longer focal length than the eye piece to increase magnification and to allow the maximum light to pass in to the telescope.
28. Astronomical telescope is not used to view the objects on the earth but is used for viewing and studying the heavenly bodies.
Ans- The telescope is used to view and study the heavenly bodies as the image of distant object formed by the telescope is inverted wrt. The object and lies exactly at the eyepiece lens forming final image at infinity but not the objects of the earth.
29. The power of lens is measured as the reciprocal of focal length.
Ans- The power of lens is the degree of divergence or convergence of light rays. The lens with long focal length can deviate less where as with short focal length can deviate more. So, the power of lens is measured as the reciprocal of focal length.
30. The combination of lenses of done.
Ans- A number of lenses are combined together to increase the sharpness of the image. The optical instruments in which lenses are combined are camera, microscope, telescope.
For combination of lenses P=P1+P2
31. How is a photograph prepared after producing image in the photographic film?
Ans- After producing images in the photographic film, the film is developed into a negative by reacting with chemicals in a dark room. This process is called developing and the film thus developed is called negative. The negative is then printed on a photographic paper and hence a photograph is obtained.
32. The photograph is developed in dark room.
Ans- The light will mess up the film. If the film is developed in the light, the light will show on the film, as the film contains light sensitive ingredient, the salt of silver. AgBr or Agcl.
33. Red light is used for developing photo film in dark room.
Ans- Red light is used as it falls in the visible region of the spectrum, it has the longest wavelength, therefore its frequency is low and the energy also law so it does not affect the photo developing process.
34. An object placed at a distance of less than 25cm cannot be seen by normal human eye.
Ans- The maximum accommodation of a normal eye is reached when an object is located about 25cm for from the eye. After this, the ciliary muscles cannot make the eye lens bulge more. Therefore, an object placed at a distance of less then 25 cm cannot be scan clearly by a normal eye because all the power a commotion of the eye has already been uses. Thus an object placed at a distance of less then 25cm cannot be scan by normal human eye.
35. We cannot see our surrounding clearly when we enter a darkened cinema hall from bright sunshine but our vision improves after some time.
Ans- The size of pupil of our eye is small in the bright sunshine. So when we enter the darkened cinema hall, very little light enters our eyes through the small pupil and we cannot see properly. After some time, the pupil of our eye expends and become large to allow in large amount light through it and we can see clearly.
