Transcript File

M.C. KEJRIWAL VIDYAPEETH
DEPT. OF PHYSICS
LIGHT
Refraction, Dispersion and Image formation via lenses &
Worksheet
Refraction and related laws, prisms
REFRACTION
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
Phenomenon and diagram
DISPERSION
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.
Types, terms, ray diagrams
LENS
Lenses
Plano Convex
Convex
Bi Convex
Lenses
Concavo Convex
Plano Concave
Concave
Bi concave
Convexo Concave
Major Terms related to a lens
Centre of
Curvature
Principal
Axis
Optical
Centre
Principal
Focus
Focal
Length
Aperture
Real v/s Virtual Image
Real images are formed Virtual images are
by actual intersection of formed when rays of
light rays
light appear to meet
They are always inverted They 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
O
F
2F
• 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
O
F
2F
• 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
O
F
2F
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
O
2F
F
F
2F
• 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
O
2F
F
2F
• Construct a ray parallel to principal axis, and make it meet at
the lens.
F
O
2F
F
2F
• 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
O
2F
F
F
2F
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 & characteristics
Lens
Convex
Object
Image
Charecteristics
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
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
Concave
M.C. KEJRIWAL VIDYAPEETH
DEPT. OF PHYSICS
CLASS VIII, SESSION 2015-16
WORKSHEET - 1
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
Worksheet
1.
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?