LIGHTRefraction, Dispersion and Image formation via lenses &

Worksheet

M.C. KEJRIWAL VIDYAPEETHDEPT. OF PHYSICS

REFRACTIONRefraction and related laws, prisms

Refraction

The phenomena due to which a ray of light deviates from its path, at the surface of separation of two media, when the ray of light is travelling from one optical medium to another, is called refraction of light.

Refraction

Laws of Refraction (Snell’s Law)• The incident ray, refracted ray and normal at the point of

incidence lie on the same plane

• The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a pair of given media. This constant is called the Refractive Index of the second medium, with respect to the first. It’s represented by 1µ2 or 1n2

Refraction through Prism

∠i = Angle of incidence∠i1 = Angle of Emergence∠δ = Angle of Deviation

DISPERSIONPhenomenon and diagram

Dispersion

The phenomenon due to which white light splits into seven colours after passing through an equilateral prism, is called dispersion.

Note: If an equilateral prism is placed in an inverted manner adjacent to the above prism, it can recombine the colours to form white light.

LENSTypes, terms, ray diagrams

LensesLe

nses

ConvexPlano Convex

Bi ConvexConcavo Convex

ConcavePlano Concave

Bi concaveConvexo Concave

Major Terms related to a lens

Centre of Curvature

Principal Axis

Optical Centre

Principal Focus

Focal Length Aperture

Real v/s Virtual ImageReal images are formed by actual intersection of light raysThey are always inverted

Virtual images are formed when rays of light appear to meetThey are always erect

Step-wise guide for construction of ray diagrams for Convex lens

• Construct a straight horizontal line as the principal axis of the lens

• At the midpoint of the line mark the optical centre of the lens as the point ‘O’

O

• From optical centre with the focal length, mark the principal focus on the principal axis, on either side of ‘O’

F O F

• Mark the centre of curvature at a distance of 2xfocal length from the optical centre on either side and mark them as 2F

2F F O F 2F

• From optical centre construct a perpendicular dotted line which will serve as the axis of the lens and draw the lens

2F F F 2FO

• Construct the object and draw a ray from top of the object and make it pass through the point O and extend it further

2F F F 2FO

• Construct another ray parallel to the principal axis, after reaching the lens, bend the ray and make it pass through the focus. The place where the two rays meet, construct the image

2F F F 2F

O

Step-wise guide for construction of ray diagrams for Concave lens

• Construct a straight horizontal line as the principal axis of the lens

• At the midpoint of the line mark the optical centre of the lens as the point ‘O’

O

• From optical centre with the focal length, mark the principal focus on the principal axis, on either side of ‘O’

F O F

• Mark the centre of curvature at a distance of 2xfocal length from the optical centre on either side and mark them as 2F

2F F O F 2F

• From optical centre, construct a perpendicular dotted line which will serve as the axis of the lens and draw the lens

2F F F 2FO

• Construct the object and draw on ray from top of the object and make it pass through the point O and extend it further

F

2F F 2FO

• Construct a ray parallel to principal axis, and make it meet at the lens.

F 2F F 2F

O

• Draw dotted lines from the focus to the meeting point of the surface of the lens and extend it further in bold line

• At the point of intersection of the two rays, draw the image in dotted lines

F

2F F 2F

O

Quick Notes• Ray of light passing through optical centre always go undeviated

• Ray of light parallel to principal axis, meets at the focus after passing through a convex lens

• Ray of light parallel to principal axis, diverges out of the focus after passing through a concave lens

• Concave lens always forms a virtual image

• Virtual rays and images are always to be constructed in dotted lines.

Lens, image & characteristicsLens Object Image Charecteristics

Convex

At Infinity At F Real. Diminished to a point, Inverted

Beyond 2F Between F & 2F Real, Diminished, Inverted

At 2F At 2F Real, Same size as object, Inverted

Between F and 2F Beyond 2F Real, Magnified, Inverted

At F At Infinity Real, Magnified, Inverted

Between O and F On same side of object

Virtual, Magnified, Erect

ConcaveAt infinity Focus on same

side of objectVirtual, diminished to a point, Erect

Anywhere between O and infinity

On same side of object between F and O

Virtual, diminished, Erect

WORKSHEET - 1CLASS VIII, SESSION 2015-16

M.C. KEJRIWAL VIDYAPEETHDEPT. OF PHYSICS

Instructions• The worksheet consists of seven questions in the next two

slides• The worksheet is to be solved in the Physics exercise books• Students may take print outs of the questions given in the

following slides and paste in the copy• Otherwise the student may copy the questions in the exercise

book and then solve the worksheet• The last date for submission of the worksheet is 4th May, 2015

Worksheet1. Under what conditions light ray will not change its direction when it

passes from one optical medium to another optical medium?

2. A ray of light passes from medium A to medium B. The angle of incidence is 35o and angle of refraction is 20o. Which medium is denser, A or B?

3. Why refractive index of any medium is always greater than one?

4. Draw ray diagram for the image formation by a convex lens when the object beyond 2F but not at infinity.

5. Draw ray diagram for the image formation by a convex lens when the object is at principal focus.

6. The focal length of the convex lens used as magnifying glass should have preferably longer or shorter focal length.

7. Which two rays remain parallel when passed through a glass block?