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# Light-theory Physics-day-4 Target ssc cgl 2018 tier 1

Light-theory Physics-day-4 Target ssc cgl 2018 tier 1

LIGHT

Light is a form of energy which is propagated as electromagnetic waves. Light travels along a straight line. Light exhibits two natures (dual nature)

• Particle nature ….light is regarded as a packet of energy which are known as photons.
• Wave nature….light is regarded as an electromagnetic wave. Since electromagnetic waves are transverse in nature, thus light is also a transverse wave.

REFLECTION OF LIGHT

Mirror changes the direction of light that falls on it. This change of direction by a mirror is called reflection of light.

In reflection, the medium of the travelling light does not changes.

Laws of reflection

1. The incident ray, reflected ray and normal to the reflecting surface, all lie on the same plane.
2. Angle of reflection is equal to the angle of reflection.

Reflection from plane mirror

• Inverted, equal size to the object and virtual image is formed.
• Image is equidistant from the mirror as the object.

PLAYING WITH SPHERICAL MIRRORS

• The most common example of a curved mirror is a spherical mirror. If the reflecting surface of a spherical mirror is concave, it is called a concave mirror. If the reflecting surface is convex, then it is a convex mirror.
• Concave mirrors always forms real and inverted images except when the position of the object is between the focus and the pole.
• As the distance of the object from the concave lens is reduced, the size of the image increases. Size of image and the object is same at the centre of the curvature.
• Convex mirrors always form virtual and erect images. As the distance of the object from the convex lens is reduced, the size of the image increases.

REFRACTION OF LIGHT

When a ray of light travels from one medium to another, it deviates from its path. This phenomenon in which the path of light changes when it travels from one medium to another medium of different refractive index is known as refraction of light.

Laws of refraction

1. The incident ray, reflected ray and normal to the reflecting surface, all lie on the same plane.
2. For a particular colour of light, the ratio of angle of incidence and the angle of refraction is always constant. This law is also known as Snell’s law.

Critical angle

When a light propagates from denser to rarer medium, the angle of incidence for which the angle of refraction is 900 is called critical angle.

Total Internal Reflection

When the angle of incidence is greater than the critical angle, the ray returns back to its own medium. This phenomenon is known as total internal reflection.

Refraction of light through lens

You might have seen a magnifying glass. It is used to read very small print. You might have also used it to observe the body parts of a cockroach or an earthworm. The magnifying glass is actually a type of a lens. Those lenses which feel thicker in the middle than at the edges are convex lenses. Those which feel thinner in the middle than at the edges are concave lenses. Notice that the lenses are transparent and light can pass through them. It is dangerous to look through a lens at the sun or a bright light. You should also be careful not to focus sunlight with a convex lens on any part of your body. A convex lens converges (bends inward) the light generally falling on it. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens , the image formed by a concave lens is always virtual, erect and smaller in size than the object.

SUNLIGHT— WHITE OR COLORED?

There are seven colors in a rainbow, though it may not be easy to distinguish all of them. These are — red, orange, yellow, green, blue, indigo and violet. White light consists of seven colors? You might have seen that when you blow soap bubbles, they appear colorful. Similarly, when light is reflected from the surface of a Compact Disk (CD), you see many colors. On the basis of these experiences, could we say that the sunlight is a mixture of different colors?

What you have learnt

• Light travels along straight lines.
• Any polished or a shining surface acts as a mirror.
• An image which can be obtained on a screen is called a real image.
• An image which cannot be obtained on a screen is called a virtual image.
• The image formed by a plane mirror is erect. It is virtual and is of the same size as the object. The image is at the same distance behind the mirror as the object is in front of it.
• In an image formed by a mirror, the left side of the object is seen on the right side in the image, and right side of the object appears to be on the left side in the image.
• A concave mirror can form a real and inverted image.
• When the object is placed very close to the mirror, the image formed is virtual, erect and magnified.
• Image formed by a convex mirror is erect, virtual and smaller in size than the object.
• A convex lens can forms real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. When used to see objects magnified, the convex lens is called a magnifying glass.
• A concave lens always forms erect, virtual and smaller image than the object.
• White light is composed of seven colours.

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