PHYSICS Refraction Of Light

Refraction Of Light• When a ray of light , travelling through one medium

strikes obliquely the surface of an other transparent medium, then there is change in its direction in the second medium. This change is called Refraction of light.

• Light refracts at boundary( b/w two mediums ) because of change in its speed.

Refraction Of Light• If ray of light enters from rare into denser medium it

bends towards the normal. But when enters from denser into rare medium, it bends away from the normal.

airglass

• e.g. from air to glass

• Light is bent towards the normal.

incident raynormal

refracted ray

From a less dense to a denser medium

airwater

From a denser to a less dense medium• e.g. from water to air

• Light is bent away from the normal.

incident rayrefracted ray

normal

airglass

Useful words to describe refraction of light

normal

incident ray

angle of refraction

angle of incidence

refracted ray

Laws Of Refraction Of Light Incident ray, refracted ray and normal lie in the same

plane. The ratio of sine of angle of incident to the sine of

angle of refraction is constant in any two transparent mediums, which is termed refractive index (n).

Refractive index (n) =sin Li/Sin Lr = Constant

Angle of Incidence (degrees) Angle of Refraction (degrees)

0.00 0.00

5.00 3.8

10.0 7.5

15.0 11.2

20.0 14.9

25.0 18.5

30.0 22.1

35.0 25.5

40.0 28.9

45.0 32.1

50.0 35.2

55.0 38.0

60.0 40.6

65.0 43.0

70.0 45.0

75.0 46.6

80.0 47.8

85.0 48.5

 

Refraction of light

Refractive index and speed of light distance that light travel in 1 s

distance that light travel in 1 svacuum (or air)

3 108 m

water (n = 1.33)2.25 108 m

glass (n = 1.5)2 108 m

diamond (n = 2.42)1.25 108 m

diamond (n = 2.42)1.25 108 m

Refractive index

Speed of light

fastestfastest

slowestslowest

Refractive Index (n) Of Different Materials

Prism• It is transparent body (Glass) ,

commonly having three sides rectangular and two sides triangular.

Refraction Of Light Through Prism• Incident ray PQ strikes side AB of prism. Inside

prism the ray in shape of refracted ray QR, bends towards the normal XY . Then emergent ray RS coming from prism bends away from the normal LM.

Refractive Index Of Glass Prism (nG)

• (nG)=

Sin (A + Dm)

2

Sin A

2

LA = Angle of prism.

LDm = Angle of minimum deviation.

nG = Refractive index of glass prism

Refraction of light through prism

Total Internal Reflection If ray of light passes from a denser medium(

water ) into a rare medium (air ) then it bends away from the normal.

Now increasing the angle of incidence, angle of refraction also increases.

Total Internal Reflection At certain stage the angle of incidence has increased

so much that the corresponding value of angle of refraction becomes equal to 90o and refracted ray lies on the surface of denser medium.

The angle of incidence for which the angle of refraction is 90o is called the critical angle.

Total Internal Reflection Now if the value of angle of incidence is increased

so much that it becomes greater than critical angle, then there is no more refraction but whole of it is reflected back in the denser medium.

Such reflection of light is called total internal reflection.

Optical Fibres

• These are very thin, flexible glass rods, used for carrying light by total internal reflection from one end to other end. A bundle of optical fibres is called light pipe.

Optical Fibres• The glass cladding of slightly lower refractive index

than that of the core, prevents surface damage to the core fibre, otherwise would allow light to escape.

Optical Fibres (uses)

• Big advantage of an optical fibre is its small size and weight.

• Optical fibres are successfully used in internal surgery to examine body cavities and in telecommunication.

Lens & Its KindsLENS = It is a transparent body(glass),

bounded by one or two Curve surfaces.KINDS OF LENS = there are two kinds of lens

(a) Convex Lens & (b) Concave Lens

Convex Lens

It is thickest in the centre, because its boundaries are curving outwards.

As it bends light inwards( rays come together), hence it is also called Converging lens.

Its focal length is taken +ve. It forms real image.

Double convex lens behaves like two prisms with attached bases.

Concave Lens

It is thinnest in the centre, because its boundaries are curving inwards.

As it spreads light out( rays move apart), hence it is also called a diverging lens.

Its focal length is taken –ve. It forms virtual image.

Uses Of Lenses

Convex lens is used in camera,projectors,binocular,telescope,microscope,spectacle and as magnifier etc.

Concave lens is used in spectacle for removing eye defect.

Convex Lens Used As Magnifier

Astronomical Telescope

Camera Compound microscope

Spectacle

Binocular

Film Projector

Slide Projector

Over Head Projector

Images Formed By Convex Lens

Images Formed By Convex Lens

Magnification & Sign Conventions For Lenses

Optical magnification is the ratio between the size of image & size of object.

M = hi = q

ho p

M = Magnification, hi = Height of image , ho = Height of object.

Distance b/w concave lens & image (q ) is taken - ve. Distance b/w convex lens & image( q ) is taken + ve. Focal length of concave lens ( f ) is taken - ve. Focal length of convex lens( f ) is taken + ve .

Power Of Lens Power of lens is the reciprocal of the focal length

expressed in meters. Power = 1 Or P = 1 focal length (in m ) f The unit of refractive power of a lens is the

diopter( where 1 diopter = !/meter ). Converging lens has a positive power and diverging

lens has a negative power. One diopter is the power of lens of focal length one

meter. A Powerful lens deviates light rays more and thus

has a short focal length. Magnifying glass( of f=2cm) has power

diopters,while high power microscope objective(of f= 4mm) has power 250 diopters.

Human EyeIn eye image is formed on retina.In it amount of light entering into eye is

controlled by Iris.For correct and clear image ( of near or

far objects) the thickness of eye’s lens is increased or decreased by ciliary muscles.

Structure Of Human Eye

Short- Sightedness(Myopia)

Definition = In this defect of vision an eye can see clearly only near objects,

Reason = the lens of eye becomes too much convergent, or eye ball becomes too long. So the images of distant objects is formed in front of the Retina.

Remedy = For correcting this defect concave lens ( diverging lens) of suitable focal length is used.

Long- Sightedness (Hyperopia )

Definition =In this defect of vision, an eye can see clear only distant objects.

Reason = the lens of eye becomes less convergent or eye ball becomes too short.

Remedy = for removing this defect convex lens(converging lens) of suitable focal length is used.

Contact Lens

Astronomical Telescope It is used to see far objects near and clear In it the focal length of object lens is long and focal

length of eye piece is short. Objective lens forms real inverted image before eye

piece, which forms its magnified ,virtual image

Astronomical Telescope

Enlarged – Image is larger than actual object.Reduced –Image is smaller than object

Compound Microscope

It is used to see minute near object clear and magnified.

In it the focal length of objective lens is short and focal length of eye piece is long.

Objective lens forms, enlarged ,inverted image, eye piece forms its further enlarged, virtual image.

Compound Microscope

CameraIn it image is formed on sensitive

film.Light reaching the film is

controlled by shutter and also by diaphragm.

For getting correct and clear image, convex lens is moved forward or backward.

Slide Projector

Camera

Lens Equation ( Convex Lens )

[As BD= p,DF= f & DI = q.]

As Δ OID and Δ ABD are similar

AB/OI = BD/DI……………….(I)

AS Δ CDF and Δ OIF are similar

CD/OI = DF/IF

As CD = AB

Then AB/OI = DF/IF ……………(II)

Lens Equation ( Convex Lens )

[As BD= p,DF= f & DI = q.] As Δ OID and Δ ABD are similar AB/OI = BD/DI……………….(I) AS Δ CDF and Δ OIF are similar CD/OI = DF/IF As CD = AB Then AB/OI = DF/IF ……………(II)

comparing I and II we get. BD/DI = DF/IF or BD/DI = DF/DI –DF……………….(III) Put value of BD,DF and DI in equation III we get p/q =f/q-f B.C.M we get pq- pf = qf or pq=qf+ pf. Dividing both sides of above Equation by pqf we get pq/pqf = qf/pqf + pf/pqf 1/f = 1/p + 1/q (Lens Equation)