light: reflection

light

it is a form of energy

it enables us to see objects from which it comes or from which it is reflected

it travels in a straight line

how do we know that light travels in a straight line?

the fact that light casts sharp shadows of opaque objects proves that light travels in a straight line. if light could bend, it would go behind the opaque object and no shadow would be formed

how do we see?

we can only see objects from which light is coming into our eyes. the light is converted to electrical signals, which go through the optic nerve to the brain, which converts it into images

how do we see non-luminous objects?

light comes from a luminous object → bounces onto the non luminous object → bounces into the eyes

luminous and non-luminous objects

luminous: objects which emit their own light

non-luminous: objects which do not emit their own light but instead reflect light coming from luminous objects

nature of light

there are two theories for the nature of light

wave theory

particle theory [not required for this chapter]

some light phenomenons can only be explained by wave theory and some can only be explained by particle theory, hence its believed that light has a duel nature (has properties of both theories)

wave theory

according to this theory, light consists of electromagnetic waves which do not require a material medium for their propagation

the wavelengths are very small (4x10^-7m)

the speed of light waves is very high (3x10^8m/s in vacuum)

define reflection of light

it is the process of sending back the light rays which fall on the surface of an object

polished, shiny surfaces reflect more light than unpolished, dull surfaces

how are mirrors made?

silver metal is one of the best reflectors of light

thus, mirrors are made by depositing a thin layer of silver on the back of a plane glass sheet. the silver layer is then protected by a coat of red paint. the reflection of light takes place at the silver surface

terms related to reflection in plane mirrors

incident ray: it is the ray of light which falls on the mirror surface. the point where the incident ray falls on the mirror is called the point of incidence

reflected ray: it is the ray of light which is sent back by the mirror

normal: it is a perpendicular dotted line to the mirror at the point of incidence

laws of reflection of light

the reflection of light from a plane/ spherical surface takes place according to two laws;

first law: the incident ray, reflected ray and normal all lie in the same plane

second law: angle of reflection is always equal to angle of incidence

regular and diffuse reflection of light

regular reflection: it occurs from smooth surfaces, and reflects parallel beams of light in one direction. this is because all particles of a smooth surface are facing in one direction

diffuse reflection: it occurs from rough surfaces, and reflects parallel beams of light in different directions. this is because all particles of a rough surface are facing in different directions

object (in the study of light)

anything which gives out light rays (either its own or reflected by it)

image

it is an optical appearance produced when light rays coming from an object are reflected from a mirror

they are of two types;

real images

virtual images

real image

images which can be obtained on a screen are called real images

it is formed when light rays coming from an object actually meet at a point after reflection

eg: cinema screen

virtual image

images which cannot be obtained on a screen are called virtual images

it is formed when light rays coming from an object only appear to meet at a point when produced backwards (but do not actually meet)

it can only be seen in mirrors

characteristics of images formed by a plane mirror

nature: virtual and erect

size: same size as the object

position: same distance from the mirror as object

lateral inversion

in plane mirrors, the image becomes laterally inversed, that is, the right side of the object appears to be the left and the left appears to be the right. this is called lateral inversion

this happens due to the reflection of light

uses of plane mirrors

to see ourselves

fitted at blind turns of busy roads so that vehicles coming from the other side can be seen

for making periscopes

spherical mirror

it is a mirror whose reflecting surface is part of a hollow sphere of glass. they are of two types;

concave

convex

concave mirror

a concave mirror is the spherical mirror in which the reflection of light takes place at the concave surface

convex mirror

a convex mirror is the spherical mirror in which the reflection of light takes place at the convex surface

terms related to spherical mirror

center of curvature

radius of curvature

pole

principal axis

principal focus

focal length

center of curvature

it is the center of the hollow sphere of which the mirror is a part

it is denoted by ‘C’

it is located in front of a concave mirror and behind a convex mirror

radius of curvature

it is the radius of the hollow sphere of which the mirror is a part

it is denoted by ‘R’

it is equal to the distance between center of curvature and pole (CP)

pole

it is the center/ middle point of the spherical mirror

it is denoted by ‘P’

it lies on the surface of the mirror

principal axis

it is the straight line passing through center of curvature and pole

it is perpendicular (normal) to the mirror at its pole

aperture

it is the reflecting surface of the mirror

it represents the size of the mirror

principal focus (concave mirror)

it is a point on the principal axis of a concave mirror where parallel rays of light converge after reflection

since all rays do actually meet, concave mirrors have real focus

principal focus (convex mirror)

it is a point on the principal axis of a convex mirror where parallel rays of light appear to converge (but actually diverge) after reflection

it can be located by extending the diverging rays backwards so that they appear to meet behind the mirror

since all rays dont actually meet, convex mirrors have virtual focus

rules for obtaining images formed by concave mirrors

RULE 1: a ray of light which is parallel to the principal axis passes through its focus after reflection

RULE 2: a ray of light passing through the center of curvature retraces its path after reflection

RULE 3: a ray of light passing through the focus becomes parallel to the principal axis after reflection

RULE 4: a ray of light which is incident on the pole makes the same angle with the principal angle after reflection (angle i = angle r)

image formation in concave mirror when object is placed between P and F

nature of image: virtual and erect

position of image: behind the mirror

size of image: magnified

usage: shaving mirror, makeup mirror, dentist mirror

image formation in concave mirror when object is placed at F

nature of image: real and inverted

position of image: at infinity

size of image: highly magnified

usage: torches, car, headlights (as it produces a strong parallel beam of light)

image formation in concave mirror when object is placed between F and C

nature of image: real and inverted

position of image: beyond C

size of image: magnified

image formation in concave mirror when object is placed beyond C

nature of image: real and inverted

position of image: between C and F

size of image: diminished

image formation in concave mirror when object is placed at infinity

nature of image: real and inverted

position of image: at F

size of image: highly diminished

uses of concave mirror

used as shaving/ makeup mirrors to see large images

used as reflectors in torches, car headlights etc because when a light bulb is placed at the focus of a concave mirror, then it produces a powerful beam of parallel light rays

used in tv dish antennas as it focuses the tv signals (parallel) onto the antenna fixed at its focus

used in the field of solar energy to focus sun rays for heating solar furnaces

new cartesian sign convention

all distances are measured from pole

distances measured in the same direction as incident light are +

distances measured against the same direction as incident light are -

distances measured above the principal axis are +

distances measured below the principal axis are -

signs in concave mirror

object distance (u) : -

image distance (v) : - (if real) and + (if virtual)

focal length (f) : -

height of object : +

height of image : - (if real) and + (if virtual)

signs in convex mirror

object distance (u) : -

image distance (v) : +

focal length (f) : +

height of object : +

height of image : +

magnification

it is the ratio of the height of image to the height of object

• if the image is virtual and erect, then magnification is +

• if image is real and inverted, then magnification is -

if m>1 then the image is bigger than the object; if m=1 then they are the same size; if m<1 then the image is smaller than the object

rules for obtaining images formed by convex mirrors

RULE 1: a ray of light which is parallel to the principal axis appears to be coming from its focus after reflection

RULE 2: a ray of light passing through the center of curvature retraces its path

RULE 3: a ray of light appearing to go through the focus becomes parallel to the principal axis after reflection

RULE 4: a ray of light incident at the pole makes the same angle with the principal axis after reflection

image formation in convex mirror when object is placed anywhere between infinity and P

nature of image: virtual and erect

position of image: between P and F

size of image: diminished

image formation in convex mirror when object is placed at infinity

nature of image: virtual and erect

position of image: at F

size of image: highly diminished

uses of convex mirror

used in rear-view mirrors. this is because it always produces an erect image, and the image formed is diminished. thus, we get a wide field of view

used as shop security mirrors

how to distinguish between mirrors

plane mirror: will produce an image of same size

concave mirror: will produce a magnified image

convex mirror: will produce a diminished image

(this only applies to the reflection of our face)