CHAPTER 5 [LIGHT] Physics Dept. SMK Sultan Ismail 1 5.1 UNDERSTANDING REFLECTION OF LIGHT The Laws Of Reflection 1. When light strikes a surface, reflection will occur. 2. The light ray that strikes the surface of the mirror is called incident ray. 3. The light ray that bounces off from the surface of the mirror is known as the reflected ray. 4. Normal is a line perpendicular to the mirror surface where the reflection occurs. 5. The angle between the incident ray and the normal is called the angle of incidence, i. 6. The angle between the reflected ray and the normal is called the angle of reflection, r. 7. When a ray of light undergoes reflection, it obeys the laws of reflection. (a) The incident ray, the reflected ray and the normal all lie in the same plane. (b) The angle of incidence, i, is equal to the angle of reflection, r. Figure 1.1 Reflection of light on a plane mirror Plane Mirror Ray Diagram to Determine the Position and Characteristics of the Image Formed by a Plane Mirror 1. The following stapes show how to draw the ray diagram for the formation of an image by a plane mirror. (a) Consider an object O placed in front of plane mirror. (b) Measure the distance between the object O and the mirror. Measure the same distance behind the mirror and mark the positions as the image, I (c) Draw the diverging ray from a point on the image, to the corner of the eye. The rays from the image to the mirror must be dotted to show that they are virtual. (d) Finally, draw two diverging rays from the object to the mirror to meet the diverging rays from the image.
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CHAPTER 5 [LIGHT] Physics Dept. SMK Sultan Ismail
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5.1 UNDERSTANDING REFLECTION OF LIGHT
The Laws Of Reflection
1. When light strikes a surface, reflection will occur.
2. The light ray that strikes the surface of the mirror is called incident ray.
3. The light ray that bounces off from the surface of the mirror is known as the reflected ray.
4. Normal is a line perpendicular to the mirror surface where the reflection occurs.
5. The angle between the incident ray and the normal is called the angle of incidence, i.
6. The angle between the reflected ray and the normal is called the angle of reflection, r.
7. When a ray of light undergoes reflection, it obeys the laws of reflection.
(a) The incident ray, the reflected ray and the normal all lie in the same plane.
(b) The angle of incidence, i, is equal to the angle of reflection, r.
Figure 1.1 Reflection of light on a plane mirror
Plane Mirror
Ray Diagram to Determine the Position and Characteristics of the Image Formed by a Plane Mirror
1. The following stapes show how to draw the ray diagram for the formation of an image by a plane mirror.
(a) Consider an object O placed in front of plane mirror.
(b) Measure the distance between the object O and the mirror. Measure the same distance behind the mirror
and mark the positions as the image, I
(c) Draw the diverging ray from a point on the image, to the corner of the eye. The rays from the image to the
mirror must be dotted to show that they are virtual.
(d) Finally, draw two diverging rays from the object to the mirror to meet the diverging rays from the image.
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Figure 1.2
2. The characteristics of the image formed in plane mirror;
(a) ________________________________________
(b) ________________________________________
(c) ________________________________________
(d) ________________________________________
Note
- The student combs his hair with his left hand
in front of a plane mirror, but the student
appears to be using his right hand if you look
at his image in the mirror.
- Left and right are interchanged.
Curved Mirrors
1. There are two types of curved mirrors,
(a) convex mirror which curves inwards
(b) concave mirror which curves outwards
2. On a concave mirror, the rays that are parallel and close to the main axis ______________ to point F ( focal
point) and the distance FP is known as the focal length of the concave mirror.
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Figure 2: Concave mirror
3. On convex mirror, parallel ray that are close to main axis, _______________ from the surface of reflection.
The ray are seen diverge from a point F (main focus) behind the mirror. The distance FP is known as the focal
length of the convex mirror.
Figure 3: Convex mirror
4. Common terminology of curved mirror.
- A curved mirror actually a spherical mirror.
- The centre of curvature, C of a curved mirror is the centre of sphere of the mirror.
- The distance between the centre of curvature, C and the surface of the mirror is called the radius of
curvature, R.
- Pole of mirror, P is the centre point on the curved mirror.
- The focal length, f is FP (FP = ½ CP)
- The focal point, F of a concave mirror is the point on the principle axis where all the reflected rays meet
and converge. Real focus for concave mirror and virtual focus for convex mirror.
- The connecting line from the centre of curvature to point P is called the principle axis.
- Object distance, u is the distance of object from the pole of the mirror, P.
- Image distance, v is the distance of image from the pole of the mirror, P.
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Ray diagram to determine the position and characteristic of an image in a convex mirror and a concave mirror
1. The following stapes show how to draw the ray diagram for the formation of an image by a convex and concave
mirror
(a) Consider an object, O placed on the principle axis in front of a mirror. Mark the surface of the mirror, P,
C and F (CF equals to FP). OP is the object distance and FP is the focal length.
(b) Draw a line through the centre of curvature, C from the top of the object.
(c) Draw another line parallel to the principle axis to meet the surface of the mirror from the top of the
object.
(d) For a convex mirror, the parallel ray that strikes the surface of the mirror will be reflected outwards
from the focal point, F whereas the ray will be reflected towards focal point, F for a concave mirror.
(e) Point of intersection is the position of the image, I. PI is the image distance.
(a) Convex Mirror (b) Concave Mirror
2. The characteristic of the image formed can be determined, that is whether the image is,
(a) ____________________________
(b) ____________________________
(c) ____________________________
Object distance, u Ray diagram Characteristics
Object at infinity
Image is real, inverted and
diminished.
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Object slightly
further from C (u >
2f or u > C)
[animation]
____________________
____________________
____________________
Object at C (u =2f
or u = C)
[animation]
____________________
____________________
____________________
Object in between C
and F ( f < u < 2f)
[animation]
____________________
____________________
____________________
Object at F (u = f)
Virtual, upright and very large
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Object in between F
and P ( u < f)
[animation]
____________________
____________________
____________________
3. Image formed by a convex mirror always ______________, _______________ and _______________
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Application Of Reflection Of Light
1. Mirror in a meter
- Plane mirrors are normally used in electrical meter instruments such as the voltmeter, ammeter and the
galvanometer as anti-parallax mirrors to avoid parallax errors.
- Accurate reading is obtained when the image of the pointer is directly below the pointer itself.
2. Mirror in a car
- By the principle of reflection of light, a plane mirror is also used as a rear view mirror and a side mirror
of a car in order to enable the driver to see object at the back and at the side of the car.
- Parabolic mirrors are used in car headlamps as reflection.
- The word ‘ambulance’ purposely inverted laterally on an ambulance car. When a driver of car in front of
the ambulance looks in his car rear view mirror, he will able to see a readable version.
3. Periscope consists of two plane mirror inclined at an angle of 45
0. It can be used to see the top of high obstacle
such as a wall. It is also used inside a submarine.
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4. If the object is placed in front of a concave mirror between F and P, a virtual, magnified and upright image is
produced. Therefore, it is used as shaving mirror and dentist’s mirror.
5. Convex mirror are used as safety mirrors which are fixed at dangerous bends to help drivers to see vehicles that
are hidden from view.
6. The image produced by convex mirror is always virtual, smaller and upright. Therefore convex mirror are used
(a) in cars as the mirrors for driving.
(b) at sharp corners and at parking lots as a safety precaution.
(c) in supermarket for safety purposes.
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5.2 Total Internal Reflection
1. Total internal reflection is ______________________________________________ ___________________, when
the ____________________ in the optically denser medium exceeds a ________________________.
2. Critical angle is ____________________________________ where the refracted angle is ____ away from the normal
line when the ray is travel from _____________ medium to a ___________________ medium. (i.e. from glass to air ).
Critical Angle and Total Internal Reflection
When the angle of incidence in the glass is increased, the results obtained are
as follows:
Small angle of incidence, e.g. 200, there is a strong refracted ray OCD and a
week reflected ray OP in the glass.
As the angle of incidence in the glass is increased, the angle of refraction in
the air increases. At one special angle of incidence, called the critical angle, c,
the refracted ray OG is 900.
A weak reflected ray is obtained, making an angle, c with normal ON.
When the angle of incidence is greater than c no refracted ray is obtained.
Instead, a new effect is seen.
The reflected ray in the glass is now almost as bright as the incident ray in the
glass.
The glass-air boundary thus behaves as a ‘mirror’ reflecting light strongly.
This is explained the total internal reflection phenomenon. [animation]
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1. A critical angel, c is defined ______________________________________________. When incident angel is beyond
the critical angle, all light rays will be totally reflected back and no light rays will be refracted out. This is known as
____________________ ____________________________
2. Total Internal Reflection only happened under following conditions: