NCERT Textbook Questions
We cannot see objects in a dark room because there is no light to reflect from them into our eyes. However, we can see objects outside the room (if there is light outside) because light reflecting from those objects enters our eyes.
| Regular Reflection | Diffused Reflection |
|---|---|
| Occurs on smooth surfaces (like a mirror). | Occurs on rough surfaces (like paper, wood). |
| Reflected rays are parallel. | Reflected rays are scattered in different directions. |
No, diffused reflection does not mean failure of the laws of reflection. The laws of reflection hold true at every point of the rough surface. The scattering occurs because the angles of incidence vary at different points due to surface irregularities.
- (a) Polished wooden table: Regular reflection. (Surface is smooth).
- (b) Chalk powder: Diffused reflection. (Surface is rough).
- (c) Cardboard surface: Diffused reflection. (Surface has irregularities).
- (d) Marble floor with water spread over it: Regular reflection. (Water makes the surface smooth).
- (e) Mirror: Regular reflection. (Surface is highly smooth and polished).
- (f) Piece of paper: Diffused reflection. (Surface is rough at microscopic level).
- The angle of incidence is always equal to the angle of reflection (i = r).
- The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
Place a mirror on a table. Take a sheet of paper and spread it on the table in front of the mirror. Shine a beam of light from a torch through a comb onto the mirror. You will see the incident ray and the reflected ray on the paper.
Now, bend the part of the paper where the reflected ray falls. You will see that the reflected ray is not visible on the bent part. This shows that the reflected ray lies in the plane of the paper. Since the incident ray and normal also lie on the paper, it proves they all lie in the same plane.
(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.
(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with left hand.
(c) The size of the pupil becomes large when you see in dim light.
(d) Night birds have less cones than rods in their eyes.
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never
Correct Answer: (a) Always
(a) virtual, behind the mirror and enlarged
(b) virtual, behind the mirror and of the same size as the object
(c) real at the surface of the mirror and enlarged
(d) real, behind the mirror and of the same size as the object
Correct Answer: (b) virtual, behind the mirror and of the same size as the object
A kaleidoscope is made of three rectangular mirror strips lying at an angle of 60 degrees to each other to form a prism. This arrangement is fixed in a circular cardboard tube or tube of a thick chart paper. One end of the tube is closed by a cardboard disc having a hole in the centre. The other end is closed with a circular plane glass plate. On this glass plate several small pieces of coloured glass are placed to form beautiful patterns via multiple reflections.
At the junction of the optic nerve and the retina, there are no sensory cells (rods or cones). As a result, no vision is possible at that spot. This is called the blind spot.
Extra Important Questions
When an image is formed by a plane mirror, the left side of the object appears on the right in the image, and the right side of the object appears on the left. This phenomenon is known as lateral inversion.
Detailed Chapter Discussion: Light
1. Introduction
The world around us is full of colourful and beautiful things. We see mountains, rivers, trees, clouds, rainbows and flying birds. At night we see the moon and stars. But we cannot see anything in the dark. What makes things visible?
We see things only when light comes from them and enters our eyes. The light may have been emitted by the object, or may have been reflected by it.
2. Laws of Reflection
When a ray of light falls on a mirror, it gets reflected. The ray of light which strikes the mirror is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray.
First Law of Reflection: The angle of incidence (i) is always
equal to the angle of reflection (r).
∠i = ∠r
Second Law of Reflection: The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.
3. Regular and Diffused Reflection
Reflection can be classified into two types based on the nature of the surface:
A. Regular Reflection
When parallel rays of light fall on a smooth and polished surface (like a plane mirror), the reflected rays are also parallel. This is called regular reflection. Images are formed by regular reflection.
B. Diffused or Irregular Reflection
When parallel rays of light fall on a rough or irregular surface (like cardboard, wall, paper), the reflected rays are not parallel but spread out in different directions. This is called diffused reflection. It is caused by the irregularities of the reflecting surface, not by failure of laws of reflection.
Illuminated Objects: Objects that shine in the light of other objects are called illuminated objects (e.g., Moon). Objects which give their own light are called luminous objects (e.g., Sun, candle, electric lamp).
4. Multiple Reflections
A plane mirror forms only a single image of an object. But what happens if two plane mirrors are used in combination?
When two plane mirrors are kept at an angle to each other, multiple images are formed. This is because the image formed by one mirror acts as the object for the other mirror. The number of images formed depends on the angle between the mirrors.
Formula: Number of images = (360° / angle between mirrors) - 1.
Kaleidoscope
A kaleidoscope is an optical instrument based on the principle of multiple reflections. It consists of three rectangular mirror strips joined to form a prism. It creates beautiful, ever-changing patterns using coloured glass pieces. Designers of wallpapers and fabrics use kaleidoscopes to get ideas for new patterns.
5. Structure of the Human Eye
The eye is one of our most important sense organs. It enables us to see things. It has a roughly spherical shape. The main parts of the eye are:
- Sclera: The outer coat of the eye is white and tough to protect the interior of the eye from accidents.
- Cornea: The transparent front part of the eye is called cornea.
- Iris: Behind the cornea, there is a dark muscular structure called iris. It controls the size of the pupil and gives distinct colour to the eye.
- Pupil: It is a small opening in the iris. It controls the amount of light entering the eye. In bright light, it becomes small; in dim light, it expands.
- Lens: Behind the pupil is a lens which is thicker in the centre (convex lens). It focuses light on the back of the eye.
- Retina: The lens focuses light on a layer called retina. It contains several nerve cells (rods and cones).
- Optic Nerve: Sensations felt by nerve cells are transmitted to the brain through the optic nerve.
Rods and Cones:
Cones: Sensitive to bright light and sense colour.
Rods: Sensitive to dim light.
6. Defects of Vision
Some people can see nearby objects clearly but cannot see distant objects so clearly. This is called Myopia (Near-sightedness). Some people cannot see nearby objects clearly but can see distant objects clearly. This is called Hypermetropia (Far-sightedness). These defects can be corrected using suitable corrective lenses.
In old age, the eye lens becomes cloudy. This is called Cataract. It causes loss of vision, sometimes extremely. It can be treated by replacing the opaque lens with a new artificial lens.
7. Visually Impaired Persons
Some persons, including children, can be visually impaired. They have very limited vision to see things. Some persons cannot see at all since birth. Some may lose their eyesight because of a disease or an injury.
Visually impaired persons try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply.
Braille System
The most popular resource for visually challenged persons is Braille. Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821. Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each.
8. Light Concepts Recap
- Sunlight: Sunlight is referred to as white light. It consists of seven colours (VIBGYOR). Splitting of light into its constituent colours is known as dispersion. Rainbow is a natural phenomenon showing dispersion.
- Lateral Inversion: The phenomenon where left appears right and right appears left in the mirror image.
50 Key Facts: Light
Light is a form of energy that enables us to see objects. We see objects when light reflected from them enters our eyes.
The bouncing back of light rays from a surface is called reflection.
The ray of light that strikes the reflecting surface is called the incident ray.
The ray of light that bounces back from the reflecting surface is called the reflected ray.
The imaginary line drawn perpendicular to the reflecting surface at the point of incidence is called the normal.
The angle of incidence (i) is always equal to the angle of reflection (r). ∠i = ∠r.
The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
The phenomenon where the left side of an object appears as the right side in the mirror image, and vice versa.
Reflection from a smooth, polished surface where parallel incident rays remain parallel after reflection.
Reflection from a rough surface where parallel incident rays are scattered in different directions. Also called irregular reflection.
Objects that emit their own light (e.g., Sun, stars, candle flame, electric bulb).
Objects that reflect light from other sources and do not emit their own light (e.g., Moon, table, books).
An optical instrument consisting of three mirrors inclined at 60° that creates beautiful patterns through multiple reflections.
An instrument used to see objects above the line of sight (e.g., in submarines), using two plane mirrors at 45° angles.
Two mirrors kept at an angle form multiple images. The number of images increases as the angle between mirrors decreases.
Two plane mirrors placed parallel to each other form an infinite number of images.
Sunlight, also called white light, is composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
The splitting of white light into its constituent colours is called dispersion. A prism or water droplets can cause dispersion.
The transparent front part of the eye that protects it and allows light to enter.
The dark muscular structure behind the cornea that gives the eye its distinct colour and controls the pupil size.
The small opening in the centre of the iris that regulates the amount of light entering the eye.
A convex lens located behind the pupil that focuses light onto the retina.
The screen at the back of the eye where the image is formed. It contains light-sensitive nerve cells (rods and cones).
Nerve cells in the retina sensitive to dim light. They enable us to see in the dark (night vision).
Nerve cells in the retina sensitive to bright light. They enable us to see colours.
The point where the optic nerve enters the retina. It has no rods or cones, so no vision is possible here.
The nerve that transmits visual sensations (signals) from the eye to the brain.
The impression of an image stays on the retina for about 1/16th of a second. This allows illusion of motion in movies.
Muscles that hold the eye lens in position and modify its curvature (focal length) to focus on near or far objects.
The ability of the eye lens to adjust its focal length to see objects at varying distances clearly.
The minimum distance at which the eye can see objects distinctly. For a normal eye, it is about 25 cm.
Near-sightedness. A person can see nearby objects clearly but distant objects appear blurry. Corrected with concave lens.
Far-sightedness. A person can see distant objects clearly but nearby objects appear blurry. Corrected with convex lens.
A condition in old age where the eye lens becomes cloudy or opaque, leading to vision loss. Treated by surgery.
A tactile writing system used by visually impaired people. It consists of raised dot patterns.
The inventor of the Braille system. He was visually impaired himself.
Essential vitamin for eye health. Deficiency leads to night blindness. Found in carrots, spinach, milk, etc.
Inability to see clearly in dim light or at night, often caused by Vitamin A deficiency.
Animals active at night (e.g., OwI). They have large pupils and more rods in their retina to see in the dark.
An owl has a large cornea and a large pupil to allow more light. Its retina has a large number of rods and only a few cones.
An involuntary action that protects the eye from foreign objects and keeps it moist.
The angle between the extended incident ray and the reflected ray.
The image is virtual, erect, same size as object, and formed at the same distance behind the mirror.
An image that cannot be obtained on a screen. Light rays do not actually meet but appear to meet.
An image that can be obtained on a screen. Light rays actually meet at a point.
The Braille system uses 63 embossed dot patterns to represent characters.
Aids that help visually impaired people learn by touch (e.g., Braille slate, stylus).
Aids that help visually impaired people learn by hearing (e.g., audiobooks, talking calculators).
Never look directly into a laser beam as its high intensity can injure the retina.
A natural spectrum appearing in the sky after rain, caused by dispersion of sunlight by water droplets.