chapter 5 light

What is light?

A form of energy that helps us see objects

What is reflection?

Bouncing back of light from a surface

What is refraction?

Bending of light when it passes from one medium to another

What is dispersion?

Splitting of white light into 7 colours

State 1st law of reflection

Angle of incidence = Angle of reflection

State 2nd law of reflection

Incident ray, reflected ray, and normal lie in same plane

State 1st law of refraction

Incident ray, refracted ray, and normal lie in same plane

What is Snell’s law?

Ratio of speeds of light in two media is constant

What is a spherical mirror?

A mirror which is part of a sphere

Types of spherical mirrors

Concave and convex

What is a concave mirror?

Mirror that curves inward

What is a convex mirror?

Mirror that curves outward

What is Pole (P)?

Centre of mirror surface

What is Focus (F)?

Point where parallel rays meet or appear to meet

What is Centre of Curvature (C)?

Centre of the sphere of which mirror is a part

What is principal axis?

Line joining P and C

What is focal length?

Distance between P and F

Parallel ray in concave mirror

Passes through F

Parallel ray in convex mirror

Appears from F

Ray through focus in concave mirror

Becomes parallel

Ray through C

Reflects back along same path

Object beyond C image in concave mirror

Between C and F, real, inverted, small

Object at C image in concave mirror

At C, same size, real, inverted

Object between C and F image in concave mirror

Beyond C, real, inverted, enlarged

Object between F and P image in concave mirror

Behind mirror, virtual, upright, enlarged

Object at F image in concave mirror

At infinity, real, inverted, highly enlarged

Nature of image in convex mirror

Always virtual, upright, diminished

Where is image formed in convex mirror

Behind mirror between P and F

Uses of concave mirror

Shaving mirror, dentist, headlights

Uses of convex mirror

Rear-view mirror, security mirror

Why does refraction occur

Due to change in speed of light

Speed order of light

Air > Water > Glass > Diamond

What is optical density

Ability of medium to slow light

Rarer to denser bending

Towards normal

Denser to rarer bending

Away from normal

Ray along normal

No bending

Formula of refractive index

μ = speed in air / speed in medium

SI unit of refractive index

No unit

Refractive index of water

1.33

Refractive index of glass

1.50

Higher refractive index means

More bending and slower light

Why pencil looks bent

Due to refraction

What is apparent depth

Depth appears less than real

Why pool looks shallow

Refraction

Why stars twinkle

Refraction by air layers

What is mirage

Illusion of water due to refraction

What happens in glass slab

Ray emerges parallel but shifted

What is lateral displacement

Sideways shift of ray

What is deviation

Change in direction of light

Do rays remain parallel in prism

No

What is dispersion

Splitting of white light

Colours of spectrum

VIBGYOR

Which colour bends most

Violet

Which colour bends least

Red

Why dispersion occurs

Different speeds of colours

How rainbow forms

Dispersion by water droplets

Why convex mirrors used in cars

Wide field of view

Why concave mirrors used in headlights

Produce parallel beam

What is a real image

Formed by actual meeting of rays and can be obtained on screen

What is a virtual image

Formed by apparent meeting of rays and cannot be obtained on screen

Which mirror forms real image

Concave mirror

Which mirror always forms virtual image

Convex mirror

What is lateral inversion

Left appears right and right appears left

Relation between focal length and radius

f = R/2

What happens when object moves from infinity

Image moves from F towards mirror

Object at infinity in concave mirror

At F, very small, real, inverted

Object at infinity in convex mirror

At F behind mirror, very small, virtual

What is optically denser medium

Medium where light travels slower

What is optically rarer medium

Medium where light travels faster

Does denser mean physically dense

No, it refers to speed of light

Why emergent ray is parallel in glass slab

Equal and opposite refraction cancels deviation

Does glass slab cause dispersion

No

Full form of VIBGYOR

Violet Indigo Blue Green Yellow Orange Red

Why violet bends more than red

Higher refractive index

Twinkling is due to

Refraction

Mirage occurs due to

Refraction in layers of air

If speed decreases what happens to refractive index

It increases

If refractive index is 1 what does it mean

Speed same as air or vacuum

Can convex mirror form real image

No

Can concave mirror form virtual image

Yes when object is between F and P

How many rays needed for diagram

Two rays are enough