LIGHT

IMPORTANT TERMS

Light: - It is a form of electromagnetic wave which is responsible for sense of sight.         

Mirror: - Mirror is a highly polished surface that reflects light and produces an image of whatever is placed in front of it.

Lens: - Lens is made of two transparent surfaces in which at least one surface is curved. It acts as a refracting medium.

Reflection of light: - Bouncing back of a beam of light after hitting any surface is known as reflection of light.

Refraction of light: - Bending of light (change in the direction) of light occurs when it passes from one medium to other. This phenomenon is called refraction of light.

Virtual image: - The image which cannot be obtained on screen is called virtual image.

Real image: - The image which can be obtained on screen is called real image.

Laterally inverted: - The interchange of left and right sides of image is called laterally inverted. The right side of object appears as its left side and vice versa.

Angle of incident: - It is angle between incident ray and normal.

Angle of reflection: - Angle between reflected ray and normal is called angle of reflection.

 

LIGHT

Light is a form of energy which produces in us the sensation of vision. Light always travels in a straight line, it is called rectilinear propagation of light. Light can form shadow.

REFLECTION OF LIGHT

The phenomenon of bouncing back of light after falling on a shiny surface is called reflection of light. We see images in mirror or water due to reflection of light. There are two types of reflection: (A) Regular reflection and (B) Diffused reflection. When light reflected from a smooth surface is called regular reflection while diffuse reflection is reflection of light by rough surfaces.

General terms related to reflection

• Incident ray: It is a ray of light that strikes a surface.
• Reflected ray: It represents the light reflected by the surface.
• Angle of incidence: It is the angle between the incident ray and the normal.
• Angle of reflection: It is the angle between reflected ray and the normal.
• Normal: The normal is a line perpendicular to surface at a point where the incident rays reflect.

MIRROR

It is a highly polished surface which is smooth enough to reflect a good fraction of light incident on it. There are two types of mirror: (A) Plane mirror and (B) Spherical mirror.

Plane mirror

It has a flat reflective surface. For light rays striking a plane mirror, the angle of reflection always equals to the angle of incidence.

Spherical mirror

Those mirrors whose reflecting surfaces are spherical are called spherical mirror. These mirrors have curved surfaces. There are two types of spherical mirrors: (A) Concave mirror and (B) Convex mirror.

Concave mirror

If the reflective surface of a spherical mirror is con’cave’ or inward, it is called concave mirror.

Convex mirror

In convex mirror the reflective surface of a spherical mirror is convex or outward, i.e. bulged or protruded.

IMAGE

It is the point where light rays meet or appear to meet. Image can be of two types: (A) Real image and (B) Virtual image

Real image

It is formed when the rays of light after reflection or refraction REALLY meet at some point.  It is always inverted. It can be obtained on screen.

Virtual image

Virtual image is formed when the rays of light after reflection or refraction APPEAR to meet at a point. It is always erect. It cannot be obtained on screen.

CHARACTERISTICS OF IMAGE FORMED BY MIRROR

Image formed by plane mirror

• The image is virtual and erect.
• The image is laterally inverted.
• The image is of same size as the object.
• The image is formed at the same distance as object is placed from the mirror.
• It is mainly used as looking glass.

Image formed by concave mirror

• When object is placed very close to the mirror (between focal point and mirror), the image formed is virtual and erect and magnified.
• When object is placed beyond the focal point, the image formed is always real and inverted.

Uses of concave mirror

• It is used by ENT and dentist to get enlarged images.
• It is used as reflectors of torches and headlights of vehicles because it allows light to spread out to infinity and cover large area in the front.

Image formed by convex mirror

The image formed by convex mirror is erect, virtual and smaller in size than that of object.

Uses of convex mirror

Since convex mirror covers a larger area in comparison to plane mirror, it is used as rear view mirror of vehicle, as reflectors at sharp turn or blind corner. It is also used in super markets, stores and ATM centers as a security measure.

LENS

It is a transparent curved device that focuses or disperses a light beam by means of refraction. Lens is made of two transparent surfaces in which at least one surface is curved. There are two types of lenses: (A) Concave lens and (B) Convex lens.

Concave lens

• It is thin in the middle and thicker at its edges.
• It is diverging lens because rays of light that passes through the lens is spread out.
• Image formed by concave lens is always erect, virtual and smaller than the object.
• It is used in spectacles to correct short sightedness.

Convex lens

• It is thick in the middle and thinner at its edges.
• It is converging lens because rays of light that passes through to meet at one point or appear to meet.
• Image formed by convex lens is usually real, inverted and smaller than the object. When the object is kept too close to the convex lens, the image is erect, virtual and larger than the object.
• It is used in spectacles to correct far sightedness.
• It is also used as magnifying glass in microscope, camera, and telescope.

COMPONENTS OF WHITE LIGHT

The white light (visible light) is composed of seven colours: Violet, Indigo, Blue, Green, Yellow, Orange and Red. VIBGYOR is used as acronym to remember colour sequence. When white light passes through a prism, it splits into its component colours. The pattern of the coloured bands obtained on the screen is called spectrum. Violet colour has the shorter wavelength and therefore it bends the most and forms the innermost part of the spectrum. Red colour has the maximum wavelength and therefore it bends the least and forms the outermost part of the spectrum.

 This is the reason rainbows show all the colours of the visible spectrum.

Rainbow

Rainbow forms in the sky due to dispersion of sunlight after the rain. It is seen as large arc consist of seven colours, i.e VIBGYOR.

Newton’s Disc

It is a circular disc on which seven colours of the rainbow are painted in seven different segments. When the disc is rotated at certain speeds, the colours get mixed together and it appears to be white. This proves that these seven colours make the white light. Since Newton was the first to make this therefore it is called Newton’s Disc.

 

INTERESTING FACTS

• The word ‘AMBULANCE’ is written in laterally inverted form on the front of the ambulance. This is deliberately done so that the driver in a vehicle ahead can easily read the word properly so that he can provide way to the ambulance.
• Rain drop acts like a prism and forms a rainbow.

• The image formed in plane mirror appears exactly like the letters itself for English alphabets like A, H, I, M, O, T, U, V, W, X and Y.
• The image distance is always equal to the object distance in case of plain mirror.
• Concave mirror and convex lens are converging in nature.
• Convex mirror and concave lens are diverging in nature.
• Images formed by convex mirror and concave lens are always erect, virtual, and smaller in size.