Light

According to the law of reflection

The angle of incidence = Angle of reflection

Let’s draw a figure for this question –

According to the given question, incident ray is making an angle of 30⁰ with the plane mirror ()

So angle of incidence ∠i + 30⁰ = 90⁰

Angle of incidence ∠i = 60⁰

From law of reflection, Angle of reflection ∠r = 60⁰

According to the law of reflection

The angle of incidence = Angle of reflection

Let’s draw the figure -

According to question reflected ray is making an angle of 90⁰ with the incident ray.

So angle of incidence + angle of reflected = 90⁰

Since both angle of incidence is equal to angle of reflected

Angle of incidence + angle of incidence = 90⁰

Angle of incidence = 45⁰

We are having only one plane mirror as per given in the question. For every incident ray, there is only one reflected ray. There will be only one reflected ray for a single ray falling on a plane mirror.

(a) For rough surface irregular reflection or diffused reflection takes place.

(b) For smooth surface regular reflection takes place.

Image formed by an object in a plane mirror is virtual because the image from behind the mirror and we cannot take this image on a screen.

Virtual image is formed by an extension of reflected ray where the ray never meets in real.

That is a reason virtual image cannot be taken on screen.

Law of the reflection means the angle of incidence is equal to the angle of reflection. For irregular reflection law of reflection does not hold true.

Periscope is used by a soldier while sitting in a bunker to see the things outside. Periscope make use of two plane mirror.

Additional Information:

Periscope: A periscope is an instrument which helps in viewing the objects which are not in the line of sight of the observer. It is used in the submarines to look at objects above the water surface.

The principle behind periscope is reflection between two parallel plane mirrors. It consists of long cylindrical hollow tube bent at right angles and two plane mirrors are kept parallel and facing each other. The mirror is fixed at 45 degrees with the framework at the top and the bottom of the tube.

Let see the simple structure of periscope –

Infinite number of the image is formed when an object is placed between two parallel plane mirrors facing each other.

When two plane mirror placed at an angle of 90⁰ to each other-

Here in the above diagram we suppose incidence angle is.

Kaleidoscope is based on the phenomenon of multiple reflections when plane mirror is place together making some angle.

Sunlight is the combination of seven components. When sunlight is allowed to pass through the glass prism it blends and gets split into seven colours. All the seven colour has different wavelength and while entering through a prism it bends (due to refraction) and angle of bending is different for all the colour. And while coming out from the prism light again get bend and we can see the entire different colour. This is also called Dispersion means spreading of white light into its full spectrum.

Human eye have crystalline lens called convex lens.

Blind spot

The phenomenon which is responsible for seeing movies in a cinema hall is Persistence of vision.

The pupil is the part of your eye that controls how much light gets in. When we enter in an area of bright light, size of pupil gets smaller (contract) because to control the amount of light.

Additional information:

While entering the dark room pupil becomes large (expands) so that more and more light can enter the eye. Size of the pupil is controlled by iris. Thus we can say that iris controls the amount of light entering into the eye.

Cone cell

Our eye contains two types of cells – cone cell and rod cell.

Cone cell is responsible for the colour and it is sensitive to bright light.

Rod cell is sensitive to dim light.

Braille (Braille is named after its creator- Louis Braille)

Braille is a system of touch reading and writing for blind persons in which raised dots to represent the letters of the alphabet. There is a rectangular block called cells that represent a certain alphabet or number or another symbol. Each cell contain risen tiny dot and different combination of this dot is used to represent a different symbol.

Let take one example –

Louis Braille is a creator of Braille. He uses 3×2 dot in each cell to represent the alphabet.

Here is an image that shows how to represent ‘A’ in Braille (only one dot is raised) –

The blind people read this ‘A’ by moving the hand from left to right.

It is not possible to represent the whole symbol from 6 dots so in present Grade 3 Braille is used to which has maximum of 63 different available cells.

Best way to learn and write Braille system is - The Hadley Institute for the Blind and Visually Impaired.

The pupil is the part of your eye that controls how much light gets in. When we enter in an area of bright light, size of pupil gets smaller (constrict) because to control the amount of light.

Additional information:

In entering the dark room pupil become large (expand) so that more and more light can enter. Size of the pupil is control by iris. Thus, we can say that iris controls the amount of light entering into the eye.

Cone cell

Our eye contains two types of cells – cone cell and rod cell.

Cone cell is responsible for the colour and it is sensitive to bright light.

Rod cell is sensitive to dim light.

A rainbow is a spectrum of sunlight. By seeing a rainbow we come to know that sunlight is consist of any colours.

We can say that sunlight = (Red + orange + yellow + green + blue + indigo + violet) here we only consider the basic colour.

(a) Iris control the amount of light entering the eye.

(b) Lens converges (focus) light ray on a layer called retina.

(a) Cone cell are sensitive to bright light.

(b) Rod cell are sensitive to dim light.

Mango (fruit), Watermelon (fruit), green vegetable and Carrots.

Speed of light in vacuum is 3×10⁸ m/s.

Given the angle between the two plane mirror = 60⁰

Number of the image formed =

So number of image will be =

= 6-1

= 5

The pupil regulates the amount of light entering the eye. However, the size of the pupil is controlled by the Iris. So, we can say that the iris controls and regulates the amount of light entering the eye

Iris control the size of the pupil.

A periscope is an instrument which helps in viewing the objects which are not in the line of sight of the observer. It is used by the submarines to look at objects above the water surface.

The principle behind periscope is reflection between two parallel plane mirrors. It consists of long cylindrical hollow tube bent at right angles and two plane mirrors are kept parallel and facing each other. The mirror is fixed at 45 degrees with the framework at the top and the bottom of the tube.

Let see the simple structure of periscope –

We use two mirrors in a simple periscope.

Myopia or Nearsightedness is one defect of the eye. (A person sees near objects clearly while distant objects appear blurred)

Light from a distant object forms an image before it reaches the retina. This could be because the eye is too long, or the cornea or crystalline lens is too strong.

Let see in figure-

Myopia is corrected by using a concave (diverging) lens of appropriate power. Property of concave lens is to divert the light which helps in forming an image on the retina.

Persistence of vision is a phenomenon of the eye by which an image is thought to persist for about one-sixteenth part of a second on retina after the actual image is formed.

It is the ability eyes to keep seeing the object an image for a fraction of a second after the object has disappeared from view. For example, while seeing a movie which is a collection of the frame (frame my photo). Each frame (photo) is moving so fast that our eye cannot differentiate between two consecutive photos. When the speed of frames moving is reduced to below 16 frames (photo) per second, we can easily differentiate between two consecutive images.

While seeing the above moving photo our eye persist as a movie clip.

Lateral inversion means the reversal of image i.e. left side of the object to appear right side in image and right side of the object appear left side in the image. For example letter, ‘b’ and its image formed by a plane mirror is ‘d’ (lateral inversion).

Example:

Source of light means any object which continuously emits light by the natural or artificial way. For example, the SUN is a natural source of light, the bulb is an artificial source of light (convert one form of energy to light energy).

Lateral inversion means the reversal of image i.e. left the side of the object to appear right side in image and the right side of the object appear left side in the image.

Let suppose the object is placed in front of the plane mirror. Its image will be formed behind the mirror and front side of the object & front side of image will face each other. This is due to the lateral inversion.

Let consider an object be in a rectangle shape (ABCD)-

By seeing the image (A’B’C’D’) point B’ appear in the leftmost and an object point B is at right side (lateral inversion) same with all the point of the rectangle.

Optic nerve: Also called cranial nerve II (second cranial nerve). The optic nerve is present in the back of the eye. The function of the optic nerve is to transfer visual information from the retina to the vision centers of the brain. The information is carried in the form of electrical impulses.

Retina:

The retina is the membrane (tissue) that covers most of the inner surface of the eyeball. It contains hundreds of millions of light-sensitive cells, called cones and rods. Rod and cone cells are also called receptors (or photoreceptors). When a ray of light is falling on the retina, the cells present over the retina get activated and convert it into nervous signals (electrical impulse) which are transported to visual centers in the brain by the optic nerve.

Here is a diagram showing major part of human eye -

Regular reflection of light takes place.

When a ray of light fall on a surface and angle of incidence is equal to the angle of reflection (law of reflection hold true) we called regular reflection.

In case of polished wooden table law of reflection hold true because the surface is polished (smooth). Thus there will be a regular reflection.

Diffuse reflection of light takes place.

When a ray of light fall (strike) on the surface of Chalk powder, reflected ray get scatted in a different direction with a different angle. So, the law of reflection doesn’t hold true. Thus, diffuse reflection.

Diffuse reflection of light takes place.

Surface of cardboard in not smooth. Law of reflection doesn’t hold.

Regular reflection of light takes place.

In mirror (any type of mirror can be plane or concave or concave) law of reflection hold. So regular reflection takes place.

Regular reflection of light takes place.

The surface of the marble floor is covered with water and on the water surface law of reflection hold true.

Additional information:-

When the light fall on the surface of the water some part gets reflected and some part get refracted.

Both laws of reflection and law of refraction hold true for water.

Diffuse reflection of light takes place.

The surface of paper is not smooth. Law of reflection doesn’t hold.

Additional information:-

If you are thinking the paper is smooth than just remember - it is impossible to write on a smooth surface. Smooth surface means no friction. So, on the surface of mirror, we can consider as friction = 0. If the paper has zero friction that implies we cannot write on the paper.

Let start drawing the diagram before start drawing we must know where the observer is placed.

Here point object is A and the ray of light from the object fall on the plane mirror. It gets reflected, here one incident ray strike at a perpendicular to the mirror and gets reflected along the same path (i₁=r₁=90⁰ at point B) and five ray strike at a different angle (point B, C, D, E, F, G). In each case, the law of reflection holds true so –

At point C i₂=r₂, at point D i₃=r₃, at point E i₄=r₄, at point F i₅=r₅, at point G i₆=r₆

Now extend all the reflected ray in the backward direction. The point (A’) where all the line meet is called Image point (A’).

The image formed by a plane mirror is virtual means we cannot take on a screen. (We cannot place screen behind the mirror)

Characteristics of the image –

1) The image formed by a plane mirror is a virtual image. (i.e. cannot be formed on a screen)

2) The image formed by a plane mirror is an erect image. (Upside)

3) Size of image and object is the same.

4) The distance between image & mirror is the same as the distance between object & mirror.

5) The image formed is a laterally inverted image i.e., right-hand side of the object seems to be the left-hand side and vice-versa.

According to Law of Reflection,

1- The angle of the incident light ray is equal to the angle of the reflected light ray:-

When a ray of light strikes on the smooth surface it gets reflected. We draw a perpendicular line at the point of incidence, this line is called normal ray. The angle between the incident ray and normal ray is called angle of incident and angle between the reflected ray and normal ray is called angle of reflection. Both the angle is equal.

2- The incident ray, the normal ray at the point of incidence and the reflected ray all lie in the same plane. (the same plane means the same side of the mirror)

In the above figure, at point O all the three rays normal, incident and reflected are coinciding i.e. lies in the same plane.