1. Refraction, Dispersion and Image formation via lenses & Worksheet
M.C. KEJRIWAL VIDYAPEETH DEPT. OF PHYSICS
LIGHT
Refraction, Dispersion and Image formation via lenses & Worksheet

2. Refraction and related laws, prisms

3. 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.

4. Refraction

5. 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

6. Refraction through Prism
∠i = Angle of incidence
∠i1 = Angle of Emergence
∠δ = Angle of Deviation

7. Phenomenon and diagram
DISPERSION

8. 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.

9. Types, terms, ray diagrams
LENS

10. Lenses Lenses Plano Convex Bi Convex Convex Concavo Convex
Concave
Plano Concave
Bi concave
Convexo Concave

12. Major Terms related to a lens
Centre of Curvature
Principal Axis
Optical Centre
Principal Focus
Focal Length
Aperture

13. Real v/s Virtual Image
Real images are formed by actual intersection of light rays
They are always inverted
Virtual images are formed when rays of light appear to meet
They are always erect

14. 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

15. 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 F

16. From optical centre construct a perpendicular dotted line which will serve as the axis of the lens and draw the lens
2F F F F O

17. 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 F O

18. 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 F O

19. 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

20. 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 F

21. From optical centre, construct a perpendicular dotted line which will serve as the axis of the lens and draw the lens
2F F F F O

22. 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 F O

23. Construct a ray parallel to principal axis, and make it meet at the lens.F
2F F F O

24. 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 F O

25. 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.

26. Lens, image & characteristics
Object
Image
Charecteristics
Convex
At Infinity
At F
Real. Diminished to a point, Inverted
Beyond 2F
Between F & 2F
Real, Diminished, Inverted
At 2F
Real, Same size as object, Inverted
Between F and 2F
Real, Magnified, Inverted
Between O and F
On same side of object
Virtual, Magnified, Erect
Concave
At infinity
Focus on same side of object
Virtual, diminished to a point, Erect
Anywhere between O and infinity
On same side of object between F and O
Virtual, diminished, Erect

27. M.C. KEJRIWAL VIDYAPEETH DEPT. OF PHYSICS
CLASS VIII, SESSION
WORKSHEET - 1

28. 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

29. Worksheet
Under what conditions light ray will not change its direction when it passes from one optical medium to another optical medium?
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?
Why refractive index of any medium is always greater than one?
Draw ray diagram for the image formation by a convex lens when the object beyond 2F but not at infinity.

30. Draw ray diagram for the image formation by a convex lens when the object is at principal focus.
The focal length of the convex lens used as magnifying glass should have preferably longer or shorter focal length.
Which two rays remain parallel when passed through a glass block?