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Chapter 5: Chapter 5: LightLight

Form 4Form 4

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PhysicsNext >

The study of The study of mattermatter

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Objectives: Objectives: ((what you will learnwhat you will learn)) 1) understanding Light

2) understanding the Laws

Physics: Chapter Physics: Chapter 55

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Light ReflectionLight Reflection

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Light travels in straight lines.When light is incident on a surface, it is reflected.

Laws of reflection of lighta. The incident ray, the reflected ray and the normal are in the same plane.b. The angle of incidence is equal to the angle of reflection.

Characteristics of images formed by plane mirrora. Same size as objectb. Virtualc. Laterally invertedd. Upright.e. The image is as far behind the mirror as the object is in front

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RefractionRefractionWhen light travels from one medium to another medium of different optical density, its speed changes. This causes the change in its direction and the light bends. The bending of light is called refraction.Laws of refractiona. The incident ray, the refracted ray and the normal lie on the same plane.b. The ratio of sine of angle of incidence to sine of angle of refraction is a constant (Snell’s Law).When light travels from an optically less dense to a denser medium, it bends towards the normal.When light travels from an optically denser to a less dense medium, it bends away from the normal.

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RefractionRefractionRefractive index of medium X may be found by the following:

n =

Sine (angle of incidence)

Sine (angle of refraction)

n =

Velocity of light in air Velocity of light in medium

X

n =

Actual depth Apparent depth

n =

1Sine C , C = critical

angle

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MirageMirageMirageA naturally-occurring optical phenomenon, in which light rays are bent to produce a displaced image of distant objects or the sky.

The interpretation of the image is up to the fantasy of human mind, and is usually mistaken for a small puddle of water.

The word comes to English via the French ‘mirage’, and from the Latin ‘mirare’, meaning 'to appear, to seem'.

It has the same root as for mirror. Like a mirror, a mirage shows images of things which are elsewhere.

The principal physical cause of a mirage, however, is refraction rather than reflection.

It is also caused by the phenomenon of total internal reflection which is the result of refraction.

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MirageMirageInferior MirageCold air is denser than warm air, and has a greater refractive index. As light passes from colder air above to warmer air below it bends away from the normal, resulting in an inferior image for the sky above.

Superior MirageWhere the air near the ground is cooler than that higher up, the light rays will curve downwards, producing a superior image. Superior mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature.

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Internal Internal ReflectionReflectionCritical angle is angle of incidence when

light travels from denser medium to less dense medium and angle of refraction is 90o.

C

Normal

Less dense medium

Denser medium

When light travels from denser to less dense medium and the critical angle is exceeded, total internal reflection takes place.Application of total internal reflection: optical fibresA light ray enters fibre at an angle > critical angleTotal internal reflection takes place along whole length of the fibre.

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LensLensThere are 2 types of lenses: convex and concave lensesConvex lens: the point where refracted rays meet is called focal pointConcave lens: the light rays seem to be diverged from a single point, the point is the focal pointFocal length (f): distance from optical centre to focal pointPower of lens =

1f (in metre) , unit is dioptre

(D)A real image can be formed on a screen. A virtual image cannot be formed on a screen.

Optical instruments which use lenses:Camera PhotocopierMagnifying glass Slide projectorMicroscope Telescope

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LensLens

1 1 1 u v f

+ =Lens equation:

v uMagnification of lens

=

Object distance (u)

Image distance (v)

Characteristics of image

Uses

u < f Same side as object

Virtual, upright, enlarged

Magnifying glass

u = f At infinity Virtual, upright, enlarged

Telescope

f < u < 2f v > 2f Real, inverted, enlarged

Slide projector

u = 2f v = 2f Real, inverted, same size

Photocopier

u > 2f F < v < 2f Real, inverted, diminished

camera

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SummarySummary

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What you have learned:What you have learned:1.1. Understanding Understanding LightLight

3.3. Using theUsing the Laws Laws

Thank YouThank You

2.2. Reflection Reflection andand Refraction Refraction