Reflection and Refraction of Light

Lecture Outline

Chapter 28:
Reflection and
Refraction

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This lecture will help you understand:
• Reflection
• Principle of Least Time
• Law of Reflection
• Refraction
• Cause of Refraction
• Dispersion
• Rainbows
• Total Internal Reflection
• Lenses
• Lens Defects

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Reflection

• We say light is reflected when it is

returned into the medium from which it
came—the process is reflection.

• When light illuminates a material,

electrons in the atoms of the material
move more energetically in response to
the oscillating electric fields of the
illuminating light.

• The energized electrons re-emit the light

by which you see the material.

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Principle of Least Time

• The idea that light takes the quickest path in

going from one place to another is called
Fermat's principle of least time.

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Principle of Least Time, Continued

• Finding the shortest time for light to go from A to B by

reflecting off the mirror

• Construct, on the opposite side of the mirror, an artificial

point, which is the same distance "through" and below
the mirror as the point B is above the mirror.

• The shortest distance between A and this artificial point

is a straight line.

• This straight line intersects the mirror at a point C, the

precise point of reflection for least time from A to B.

Multiple Choice

In Fermat's least principle which of the following is the shortest travel for light?

• distance

• time

• imaginary

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Law of Reflection

• Angle of incidence

– Angle made by the incoming ray and the perpendicular

• Angle of reflection

– Angle made by the reflected ray and the perpendicular

• Normal

– Imaginary line perpendicular to the plane of the reflecting

surface

– Lies in the same plane as the incident and reflected rays

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Law of Reflection, Continued

• Law of reflection

– The angle of reflection equals the angle of

incidence.

Multiple Choice

According to the Law of Reflection

• The angle of incidence is perpendicular to the normal

• The angle of incidence is perpendicular to the angle of reflection

• The normal is perpendicular to the angle of reflection

• The angle of incidence and the angle of reflection are equal

Multiple Choice

The Law of Reflection applies to

• light

• sound

• both a and b

• neither a or b

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Law of Reflection, Continued-1

• Virtual image

– is same size as object, formed behind a mirror, and

located at the position where the extended reflected
rays converge.

– is as far behind the mirror as the object is in front of

the mirror.

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Law of Reflection, Continued-2

• Plane mirror

– Note: the only axis reversed in an image is

the front-back axis.

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Law of Reflection, Continued-3

• Shape of mirror forms a different virtual image.

– Convex mirror (that curves outward): virtual image is

smaller and closer to the mirror than the object.

– Concave mirror (that curves inward): virtual image is

larger and farther away than the object.

Multiple Choice

Which type of mirror will create a larger image?

• convex

• concave

• double

Multiple Choice

Light reflecting from a smooth surface, like a mirror, undergoes a change in

• frequency

• speed

• wavelength

• none of the above

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Law of Reflection, Continued-4

• Diffuse reflection

– When light strikes a rough or irregular surface and

reflects in many directions

– An undesirable circumstance is the ghost image that

occurs on a TV set when TV signals bounce off
buildings and other obstructions.

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Law of Reflection, Continued-5

• Different road surfaces determine amount of

diffuse reflection
– Rough road surface—because of diffuse

reflection, see road ahead of car at night.

– Wet road surface is smooth—because of less

diffuse, reflection, difficult to see.

Multiple Choice

Diffuse reflection occurs when the sizes of irregular surfaces

• small compared with the wavelength

• large compared with the wavelength of reflected radiation

• both a nd b

• none of these

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Refraction

• When light bends in going obliquely from one

medium to another, we call this process
refraction.

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Refraction, Continued

• Refraction occurs to

minimize the time taken
by light to travel from A to
B.

• Just as if you wanted to

save someone from
drowning, the quickest
path would not be a
straight line – it would be
the dashed path shown.

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Refraction, Continued-1

• Light follows a less inclined path in the glass.

– Light travels slower in glass than in air, so it

minimizes the time it spends in the glass.

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Refraction, Continued-2

• Light rays pass from air into water and water into

air.
– Pathways are reversible for both reflection

and refraction.

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Refraction, Continued-3

• Refractive index:
• Index of refraction, n, of a material

– indicates how much the speed of light differs

from its speed in a vacuum.

– indicates the extent of bending of rays.
– ratio of speed of light in a vacuum to the

speed in a material.

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Refraction, Continued-4

• Refractive index (continued):

– In equation form:

‒ Medium with a high index means high

bending effect and greatest slowing of light.

Multiple Choice

Refracted light that bends toward the normal is light that has

• slowed down

• sped up

• nearly been absorbed

• diffracted.

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Refraction
CHECK YOUR ANSWER
Refracted light that bends toward the normal is
light that has

A.

slowed down.

Multiple Choice

Refracted light that bends away from the normal is light that has

• slowed down

• sped up

• nearly been absorbed

• diffracted

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Refraction, Continued-5

• Illusions caused by refraction

‒ Objects submerged in water appear closer to

the surface.

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Refraction, Continued-6

• Illusions caused by refraction (continued)

‒ Objects such as the Sun seen through air are

displaced because of atmospheric refraction.

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Refraction, Continued-7

• Illusions caused by refraction (continued)

‒ Atmospheric refraction is the cause of

mirages.

Multiple Choice

When light travels from one medium to another and changes speed in doing so, we call this process

• refraction

• interference

• disperson

• reflection

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Cause of Refraction

• Refraction

– Bending of light when it passes from one

medium to another

– Caused by change in speed of light

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Dispersion

• Dispersion

– Process of separation of light into colors arranged by

frequency

‒ Components of white light are dispersed in a prism

(and in a diffraction grating).

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Dispersion
CHECK YOUR NEIGHBOR
When white light passes through a prism, green
light is bent more than

A.

blue light.

B.

violet light.

C.

red light.

D.

None of the above.

Multiple Choice

When white light passes through a prism, green light is bent more than

• blue light

• violet light

• red light

• none of these