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# Reflection of light on plane surfaces, laws of reflection, image formation by plane mirror, application of reflection on plane mirror

REFLECTION OF LIGHT ON PLANE SURFACE

There are two types of reflection:

1.          Regular Reflection

2.         Diffuse Reflection or Irregular Reflection

In regular reflection, parallel rays of light incident on a smooth or polished surface are reflected as parallel rays in one direction. In diffused or irregular reflection, parallel rays of light incident on a rough or irregular surface are reflected in various directions

LAWS OF REFLECTION

The first law of reflection states that the incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane

The second law of reflection states that the angle of incidence (i) is equal to angle of reflection (r).

CHARACTERISTICS OF IMAGE FORMED BY PLANE MIRROR

1.          It is the same size as the object

2.         It is virtual

3.         It is laterally inverted

4.         It is upright

5.         It is far behind the mirror as the object is in front of the mirror

IMAGE

There are two types of image:

1.          Real image

2.         Virtual image

A real image is one that can be caught on a screen.  Light rays actually pass through real image.  A virtual image is one that cannot be caught on a screen.  It is one through which rays do not actually pass but which is nevertheless visible to the eye.

LATERAL INVERSION

The effect on plane mirror on objects placed in front of it whereby the appearance of the image looks like a reversal of the object is known as lateral inversion

IMAGES FORMED BY INCLINED MIRROR

When two mirrors are placed at an angle to each other, the number of images formed is given by:

N = (360/ Ө) – 1

N = Number of images

Ө = Angle of inclination

When Ө = 1800, the two mirrors will act as a single mirror and therefore formed only one image.  When Ө = O, the two mirrors are parallel to each other and the image of object placed between them will be at infinity.

EFFECT OF MIRROR ROTATION ON REFLECTED RAY-MIRROR GALVANOMETER

If the direction of an incident ray on a mirror is kept constant and the mirror is rotated through twice that angle.  This fact is utilized in mirror galvanometer (to measure very small electric current) and in the navigators sextant.

USES OF PLANE MIRROR

1. It is used in periscope

2. It is used in kaleidoscope

3. It is used in sextant

General Revision

1. An object is released from rest at a height of 25m. Calculate the time it take to fall to the ground? [g=10m/s2]

2   A body is projected horizontally from the top of a tower with a velocity of 20m/s. It land on a     level ground at a horizontal distance of 60m from the foot of the tower. Calculate the height of     the tower.  [g= 10m/s2]

WEEKEND ASSIGNMENT

1   Plane mirrors are used in all these except

• periscope (b) sextant (c) kaleidoscope (c) binoculars

2  Two plane mirrors are placed touching and at 600 to each other. If an object is placed      between the mirrors and viewed from above the mirrors. How many images will the eye      see?

(a) 5 images   (b) 6 images (c) 4 images (d) 3 images

3    When a ray of light is reflected from a plane surface, the angle of incidence is always equal       to the angle of  ( a) reflection (b) refraction ( c ) diffraction (d) dispersion

4   Which of the following statements is untrue?

When an image is formed in a plane mirror, the image formed will be

(a) the same size as the object(b) smaller than the object  (c) laterally inverted (d) always     virtual

5   In which of the following instrument is the image that is formed erect

• pin hole camera ( b) simple camera (c) microscope (d) periscope

THEORY

1 What do you understand by the term lateral inversion?

2 Two plane mirrors inclining at an unknown angle, forms 11 images. Find the value of the angle?