Refractions of Light
Category : 8th Class
Refractions of Light
We know that the light travels with different speed, in different mediums. Italways travels in straight line. When it travels from one medium to another, itchanges its direction. The change in the direction of light, as it passes fromone medium to another, is known as refraction of light. If it goes from rarermedium to denser medium, it bend towards the normal. If it goes from densermedium to rarer medium, it bends away form the normal.
The speed of light in air is 3 x 108 m/sec and that in the glass is 2 x108 m/sec.
The speed of light in water is 2.25 x108 m/sec.
Thus, we can define the refractive index of the medium as the ratio of speedof light in the medium, to the speed of light in the air. It is also called theSnell's law. It is given by,
\[\mu =\frac{\sin i}{\sin r}\]
Various Effects of Refractions of Light
(a) A stick immersed in water appears to be bent at the surface of thewater.
(b) The bottom of the swimming pool appears to be less deep, thanactually it is.
(c) Stars appear twinkling in the sky during the night.
(d) Appearance of the pool of water in the desert during summer, whenobserved from the long distance.
Lens
It is the transparent material having two refracting surfaces. It is of two type,concave lens and convex lens. The lens, which is thicker at the centre andthinner at the edge is called convex lens. It is also called converging lens. Onthe other hand, the lens which is thinner at the centre and thicker at the edgeis called concave lens. It is also called diverging lens.
Dotted lines are where light appears to be corning from when seen from right-hand side of the lens.
Laws of Refraction of Light
There are two laws of refraction of light:
(a) The incident ray, the refracted ray, and the normal, all lies on thesame plane.
(b) The ratio of angle of incidence and the angle of refraction is aconstant.
Formation of Imageby Convex Lens
| Position of object | Position of Image | Size of Image | Nature of Image |
| (a) Between optical centre and F | On the same side of object | Enlarge | Virtual and erect |
| (b) At F | At infinity | Highly enlarge | Real and inverted |
| (c) Between F and 2F | Beyond 2F | Larger in size | Real and inverted |
| (d) At 2F | At 2F | Equal in size to theobject | Real and inverted |
| (e) Beyond 2F | Between F and 2F | Smaller in size | Real and inverted |
| (F) At infinity | At F | Highly diminished | Real and inverted |
Martin took two different medium, one water and other milk. Heallowed the light to pass through them, and found that the speed oflight was different in both the medium. Which one of the followingis the speed of light in water?
(a) 3 x 108 m/sec
(b) 2.25 x 108 m/sec
(c) 1.5 x 108 m/sec
(d) 3.25 x 108m/sec
(e) None of these
Answer: (b)
Explanation
The speed of light in the water is 2.25 x 108 m/sec.
We know that the light under goes refraction when it passe from onemedium to another. Some times it bends towards the normal andsome times it bands away form the normal. Sam passed a beam oflight from water to the glass slab and observed the deviation of thelight. The ray of light bends_____________.
(a) Towards the normal
(b) Away from the normal
(c) No deviation
(d)At right angle
(e) None of these
Answer: (a)
Explanation
If the light pass from rarer to denser medium, it bends towards thenormal.
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