Studied by 32 people
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)
Note 
Flashcard (78)