Reflection of light

reflection

when light travelling in a medium encounters encounters a boundary leading to a second medium, part of the incident light is returned to the first medium which it

mirror

properties

how are they made

purpose/what do they do

any smooth surface can act as a mirror

can be plane or curved

made by depositing a thin film of aluminium on a polished surface

can be coated on back or front depending on type/purpose

redirect light rays and forms an image of objects

2 types of reflection (at plane surfaces)

1- regular/specular reflection

2- diffuse reflection

regular/specular reflection

when a parallel pencil of rays is incident on a highly polished surface in a single (THE SAME) direction

diffuse reflection

when a parallel pencil of rays is incident on an irregular or rough surface (eg - a non glossy white paper) it is reflected or scattered in all directions

how is diffuse reflection used in real life/day to day scenarios

helps us to detect objects in everyday experiences, for example when driving at night the road is made easily visible by diffuse reflection of the car head light beams from the irregular road surface

incident ray

the ray that is hitting/going towards the mirror surface

reflected ray

the ray that is reflected from the mirror surface

point of incidence

the point at which the incident ray hits the mirror surface and then gets regularly reflected along the path BC (reflected ray)

normal

line which is perpendicular to the reflecting surface at the point of incidence

what is i

the angle of incidence - angle between the incident ray and the normal

what is i’

angle of reflection - angle between the normal and reflected ray

what is the first law of reflection

the incident ray, reflected ray and the normal lie in one plane which is perpendicular to the (mirror surface

what is the second law of reflection

the angle of incidence (i) is equal to the angle of reflection (i’)

i = i’

this law can be proved by applying Huygen’s principle

The formation of an image by a plane mirror

S - point source of light placed in front of a plane mirror

three rays of light leaving S and are incident on the mirror at points A, B and C

the ray SA is reflected back along the direction AS

rays at B and C will travel along the reflected directions BD and CE

SA and DB extended backwards to the point S’ where they meet

At B, <SBN = i and <NBD = i’

SAS’ is parallel to BN so <ASB = i - due to alternate angles

<AS’B = i’ - due to corresponding angles

congruent triangles

have the exact same three sides and three angles

triangles SAB and S’AB:

<SAB and S’AB are = both 90 degrees

<AS’B and ASB are = as i = i’ / the angle of incidence and refraction are equal

the side AB is common for both

so SA = S’A eg if SA is 1m the image will also be formed at 1m

explain how the rays diverge from S’

ray CE appears to be diverging from S’

when viewed from in front of a mirror all the reflected rays appear to diverge from the point S’

what is S’

virtual image of S

conclusion of SA and S’A

SA = S’A

for a plane mirror the image distance behind the mirror is the same as the object distance in front of the mirror

characteristics of the image formed by a plane mirror

position, size, nature and appearance

position

the image is as far behind the mirror as the object is in front

the distance between the object and the mirror is the same as the distance between the mirror and the image, = distances

size

the image is the same size as the object

nature

image is virtual / real (depending)

appearance

we can say an image is upright, laterally inverted left to right)

eg if a child holds up their left hand in front of a mirror it looks like they are holding their right hand so is inverted

what is a sight testing chart/visual acuity chart

chart used by optometrists to check vision and usually involves the use of a plane mirror

why are plane mirrors involved/needed in the use of visual acuity charts

charts need to be placed at 6m from the patient

however most testing rooms are smaller/less than this so use plane mirrors

the chart is placed just above the patient and a mirror is placed at 3m from the patient, so the patient will view a virtual image of the chart via the mirror kept at 3m

as the distance from the mirror to the object and the mirror to the image is the same this is a total of 6m enabling the right distance from patient to chart

IMP: letters on the chart need to be printed in reverse so it appears correct way for the patient

two types of curved mirrors

concave and convex mirrors

represented by DAE

concave mirror

polished surface on the outside

convex mirror

polished surface on the inside

pole/vertex

this point (A) is the center of the of the surface/mid-point of the mirror surface

center of curvature point

this point (C) is the center of curvature of the sphere of the which the mirror forms a part

principle of axis of the mirror

this is line AC produced in both directions

what does line DE represent

the diameter or aperture of the mirror

radius of curvature

the distance between A and C is the radius of curvature of the mirror, AC = r

what is the principle focus of the mirror (F)

the point at which the reflected rays intersect: If the incident pencil of rays is limited to a narrow beam parallel to and close to the principal axis of the mirror, the reflected rays all intersect, or appear to intersect at a single point (F)

explain how point F - the principle of focus is a REAL point in a concave mirror

As the rays of light actually intersect/(converge) at this point - to prove this if we place a screen at F, a clear image of the distance object will be formed

explain how point F - the principle of focus is a VIRTUAL point in a convex mirror

As the rays only appear to diverge from it. If we place a screen at F no image will be formed

what is the distance AF

the focal length of the mirror, f