physics final revision - light and vision 2013.pdf

7/27/2019 Physics Final Revision - Light and Vision 2013.pdf

http://slidepdf.com/reader/full/physics-final-revision-light-and-vision-2013pdf 1/8

╞╡§¥ Physics SPM 2013 Chapter 5: Light and Vision

Hoo Sze Yen www.physicsrox.com of 8

CHAPTER 5: LIGHT AND VISION

These notes have been compiled in a way to make it easier for revision. The

topics are not in order as per the syllabus.

5.1 Mirrors and Lenses

5.1.1 Image Characteristics

Image characteristics are described using the following three categories:

Size Same Image is exactly the same size as the object

Magnified Image appears bigger than the object

Diminished Image appears smaller than the object

Direction Upright Image appears to be in the same direction as the object

Inverted Image appears upside down compared to object

Type Real Real images are images you can capture on a screen. Mirrors: Images are formed on the same side of the mirror as the object Lenses: Images are formed on the opposite side of the lens from the object

Virtual Virtual images are images you can see but cannot capture on a screen. Mirrors: Images are formed on the opposite side of the mirror from the object Lenses: Images are formed on the same side of the lens as the object

5.1.2 Plane mirrors

Law of light reflection:• The reflected angle is always the same as the incident angle.

• The incident ray, reflected ray, and normal line are in the same plane.

Characteristics of an image formed by a plane mirror:

Size Same

Direction Upright, laterally inverted

Type Virtual

Distance Distance of an image from the plane mirror is the same as the distance of

the object from the mirror

normal Incident ray Reflected ray





5.1.3 Curved Mirrors vs Lenses

Concave mirror Convex mirror

Also knownas

Converging mirrors Diverging mirror

Focal

lengths

Positive

E.g. f = +20cm.

Negative

E.g. f = -20cm.

Convex lens Concave lens

Also known

as

Converging lens Diverging lens

Focallengths

PositiveE.g. f = +20cm.

NegativeE.g. f = -20cm.

For both concave and convex mirrors, the focal length is half the radius; i.e. CF = FP .





Determining the Position and Characteristics of an Image with a Ray Diagram

Concave mirror

A ray parallel to the principal axis is reflected to pass through F

A ray through F is reflected parallel to the principal axis

A ray through C is reflected back along its own path

Convex mirror

A ray parallel to the principal axis is reflected as if

it came from F

A ray towards F is reflected parallel to the principal axis

A ray towards C is reflected back along its own path

Convex lens

A ray parallel to the principal axis is refracted to pass through F

A ray through F is refracted parallel to the principal axis

A ray through C travelsstraight along its own path

Concave lens

A ray parallel to the principal axis is refracted as if it came from F

A ray towards F is refracted parallel to the principal axis

A ray towards C travelsstraight along its own path

To determine the position and characteristics of an image using a ray diagram:

1. Draw two rays emanating from the top of the object to the mirror or lens, and using the guide in the tableabove, draw their reflected/refracted paths.

2. The image is produced at the intersection of the two reflected/refracted rays.





Images formed by a Concave Mirror / Convex Lens

Posit ion of

object

Ray diagram of concave

mirrors

Ray diagram of convex

lenses

Characteristics

of image

Between F and the

mirror /

lens

Virtual Upright

Magnified

At F

Virtual

Upright

Magnified

At infinity

Between F

and C/ 2F

Real

Inverted

Magnified

At C / 2F

Real

Inverted

Same size

Greater

than C / 2F

Real

Inverted

Diminished

At infinity

Real

Inverted

Diminished

Images formed by a Convex Mirror / Concave lens

Posit ion of

object

Ray diagram of convex

mirror

Ray diagram of concave

lens

Characteristics

of image

Anywhere

in front of

the mirror

or lens

Virtual

Upright

Diminished





SUMMARY OF COMPARISON OF IMAGE CHARACTERISTICS

Characteristics of concave mirrors are the same as convex lenses:

Object distance Image characteristics

u = ∞ Real Inverted Diminished

u > 2 f Real Inverted Diminished

u = 2 f Real Inverted Same Size

f < u < 2 f Real Inverted Magnified

u = f Virtual Upright Magnified

u < f Virtual Upright Magnified

Characteristics of convex mirrors are the same as concave lenses:

Virtual, Upright, Diminished

Lens / Mirror

f 2 f

Real, Inverted Virtual, Upright

Same size

MagnifiedDiminished





5.1.4 Lens Equation

f vu

111

where u = object distance [cm]

v = image distance [cm] f = focal length of lens [cm]

5.1.5 Lens Power

f P

1

OR f

P 100

where P = lens power [D]

f = focal length [m]

where P = lens power [D]

f = focal length [cm]

5.1.6 Linear Magnification

Linear magnification is the ratio of the image size to the object size.

u

v

h

hm

o

i

where m = linear magnification

hi = height of image

ho = height of object

5.1.7 Application of Lenses

Complex Microscope

Astronomical Telescope

Focal length, f

Convex lens: positiveConcave lens: negative

Object distance, u Always positive

Image distance, v If positive: real image

If negative: virtual image

f o < f e

m > 1: magnified

m = 1: same size

m < 1: diminished

f o > f e

Magnification =

e

o

f

f

Normal setting:

Length between lenses = f o + f e





5.2 Refraction and Total Internal Reflection

Light refraction is a phenomenon where the direction of light is changed when it crosses the

boundary between two materials of different optical densities. It occurs as a result of achange in the speed of light as it passes from one medium to another.

When a light ray travels from medium A

to medium B which is optically denser

than A

When a light ray travels from medium C

to medium D which is optically denser

than C

The ray of light will refract towards

normal; r < i

The ray of light will refract away from

normal; r > i

When a light ray crosses the boundary between two different mediums at a right

angle

i = 0°, r = 0°

5.2.1 Snell’s Law

Snell’s Law states that the ratio of sin i to sin r is a constant.

r

i

sin

sin= constant





5.2.2 Refractive Index

The refractive index or index of refraction of a medium is equivalent to the optical density of

a medium.

Note: A material with greater density may not necessarily have greater optical density.

The refractive index / index of refraction of a medium, n can be calculated as:

n =r

i

sin

sin

=v

c

medium,in thelightof speed

air,inlightof speed

=d

D

depth,apparent

depth,actual

=csin

1

(where c is the critical angle)

5.2.3 Total Internal Reflection

Critical angle, c is the value of the

incident angle when the refracted angle is

90°.

• When i is increased to be greater than

c, the light will be complete reflected

back into the material. No light will

be refracted.

• This phenomenon is known as total

internal reflection.

Conditions for total internal reflection:1. Light must be traveling from an optically denser medium to a less dense medium.

2. The incident angle must be greater than the critical angle.

END OF CHAPTER

physics final revision - light and vision 2013.pdf

Documents