LIGHT

          The light is a form of energy which makes objects visible to our eyes.the branch of physics which deals with nature of light, its source, properties, measurements, effects and vision is called optics.

Role of light in day to-day life

• In our daily life we see many objects.
• How do we see them? We need light see objects.
• The objects that give light are called as sources of light.
• The sun, stars, candle, torch light etc. give us light.
• When light from these objects reaches our eyes, we are able to see them.
• Such bodies are called luminous bodies.
• The light from the luminous bodies falls on the table, chair etc and reaches our eyes.
• Thus we are able to see them
• Here table, chair etc. are called non-luminous bodies.
• The body that emits light of its own is called luminous body.
• The body that does not emit light of its own is called non-luminous body.
• Moon is not a luminous body.
• It is reflects sunlight.
• Sunlight reaches the earth in 8 minute and 20 second.
• We should not see the sun directly for a long time.
• It may affect our eye sight.
• Light travels in a straight line.

Transparent and opaque objects

• We are able to see objects clearly through a glass or pure water. So glass and pure water are transparent objects.
• We cannot see objects through a stone or a ball. The objects which do not allow light to pass through them are called opaque objects. So stone and ball are opaque objects.
• The objects are not clearly seen and appear blurred. The objects which partially allow light to pass through them are called translucent objects. These are translucent objects.
• When some objects are placed in the path of light, a dark portion is formed on the opposite side of the objects. This dark portion is called shadow.

Lunar eclipse

• The sun -source of light
• The Earth -opaque object
• The Moon -screen
• When the shadow of earth falls on the moon, the moon is hidden.

Solar eclipse

• The Sun -source of light
• The Moos -opaque object
• The Earth -screen
• When the shadow of the moon falls on earth the Sun is hidden.

LIGHT

• We see a variety of object in the world around us. However we are unable to see anything in a dark room.
• On lighting up the room things becomes visible what makes things visible? During the day sunlight helps us to see object.
• An object reflects light that falls on it. This reflected light when received by our eyes, enable us to see things.
• There are a number of common wonderful phenomenon associated with light.
• In this chapter, we shall study the phenomenon of reflection and refraction of light using the straight line propagation of light.

REFLECTION OF LIGHT

Laws of reflection of light. Let us recall these laws.

• The angle of incidence is equal to the angle of reflection, and
• The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.

Regular and Irregular reflections

• Reflection from a polished surface is called regular reflection.
• Reflection from a rough (unpolished) surface is called irregular or diffused reflection.
• In the case of a rough surface, light is not reflected in one direction, it is scattered in all directions. This is called a diffused or irregular reflection.
• In there any relation between the number of images and the angle between the mirrors?

360

          Number of  images=                                    -1

                                                          Angle

• When the mirrors are placed parallel to each other, maximum number of images will be formed.
• Based on the principle of multiple reflections we can make the kaleidoscope and Periscope.

SPHERICAL MIRROR

• A spherical mirror whose reflecting surface is curved inwards is called concave mirror.
• A spherical mirror whose reflecting surface is curved outwards is called a convex mirror.
• The distances between the pole and the principal focus of a spherical mirror is called the focal length.

REFLECTION OF LIGHT BY SPHERICAL MIRROR

• The ray parallel to the principal axis, after reflection, will pass through principal focus in case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.
• A ray passing through the principal focus of concave mirror or a ray directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.

USES OF CONCAVE MIRROR

• Concave mirror are commonly used in torches, search – light and vehicles head light to get powerful parallel beam of light.
• They are used as shaving mirrors to see a large image of the face.
• The dentists use concave mirrors to see large images of the teeth of patients.
• Large concave mirrors are used to concentrate sun light to produce heat in solar furnaces.

IMAGE FORMATION BY A CONVEX MIRROR

USES OF CONVEX MIRROR

• Convex mirror are commonly used as rear-view mirrors in vehicles.
• These mirrors are fitted on the sides of the vehicles, enabling the driver to see traffic behind him/ her to facilitate safe driving.
• Convex mirror give an erect image, Also they have a wider field of view as they as curved outwards.

MIRROR FORMULA

• In a spherical mirror, the distance of the object from its pole is called the object distance (u).
• The distance of the image from the pole of the mirror is called the image distance (V).
• You already known that the distance of the principal focus from the pole is called the focal length (f).
• These is a relationship between these three quantities given by the mirror formula which is expressed as

1/v + 1/u = 1/f

REFRACTION OF LIGHT

Laws of refraction

• Refraction of light is due to change the speed light as it enters from one transparent medium to another.

Following are the laws of refraction of light

• The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
• The ratio of sine of angle of incident is a constant, for the light of given pair media. This law is also known as Snell’s law of refraction.  If I is the angle of incidence and r is the angle of refraction then

Sin i/sin r=constant

REFRACTIVE INDEX

• We know that a ray of light travels obliquely from one transparent medium into another will change its direction in the second medium.
• The refractive index of the second medium with respect to the first
• μ=Sisin r

μ = Sin i/sin r

μ =

REFRACTION BY SPHERICAL

Spherical lenses

• A transparent material bound by two surfaces, of which one or both surface are spherical, forms a lens.
• A lens may have two spherical surfaces, bulging outwards. Such a lens is called a double convex lens.
• Convex lens converge light rays. Hens it is called converge lens.
• A double concave lens is bounded by spherical surface, curved inwards.
• It is thicker at the edges then the middle. Such lenses diverge light rays and are called diverging lenses.
• A double concave lens is simply called a concave lens.

 LENS FORMULA

• This formula gives the relation between object – distance(u), image – distance (v) and the focal length (f). The lens formula is expressed as

1/v + 1/u= -1/f

MAGNIFICATION

• The magnification produced by a lens is defined as the ratio of the height of the image to the height of the object.

POWER OFLENS

• The power of lens is defined as the reciprocal of its focal length.
• It is represent by the letter P. the power P of a lens of focal length f is given by

p =

• The SI unit of power of a lens is ‘dioptre’

DISPERSION OF WHITE LIGHT BY A GLASS PRISM

• The Various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red.
• The band of the coloured component of a light beam is called its spectrum.

ATMOSPHERIC REFRACTION

• The apparent random wavering or flickering of objects seen though a turbulent stream of hot air rising above a fire.
• This wavering is thus an effect of atmospheric refraction (refraction of light by the earth’s atmosphere) on a small scale in our local environment.
• The twinkling of stars is similar phenomenon on a much larger scale.

TOTAL INTERNAL REFLECTION

When a ray of light passing from a denser medium to a rarer medium, the refracted ray is bent away from the normal.

At a certain angle of incidence angle of refraction becomes 90 degrees. Such angle is called critical angle.

If the angle of incidence is increased further, the ray will be completely reflected back ito the same medium. This is known as total internal reflection. 

HUMAN EYE

• Human eye is like a camera
• Its lens system forms an image on a light sensitive screen called retina.
• Light enters the eye through the thin membrane called the cornea.
• It forms the transparent bulge on the front surface of the eye ball.
• The crystalline lens merely provides the finer adjustment of focal length required to focus object at different distances on the retina.
• We find a structure called iris behind the cornea.
• Iris is a dark muscular diaphragm that controls the pupil.
• The pupil regulates and controls the amount of light entering the eye.
• The retina is a delicate membrane having enormous number of light sensitive cells.
• The light sensitive cells get activated upon illumination and generate electrical signals.
• These signals are sent to the brain via the optic nerves.

DEFECTION OF VISION AND RECTIFICATION

• (i) Myopia or near – sightedness

(ii)Hyper metropia or far – sightedness, and

(iii)Presbyopia

Myopia

• Myopia is also known as near sightedness.
• A concave lens is suitable power will bring the image back on to the retina and thus the defect is corrected.

Hypermetropia

• Hypermetropia is also known as far – sightedness.
• The defect either because (i) the focal length of the eye lens is too long or (ii) the eyeball has become too small.
• This defects can be corrected by using a convex lens of appropriate power.
• Eye- glasses with converging lenses provide the additional focusing power required for forming the image of the retina.

Presbyopia

• The power of accommodation of the eye usually decreases with ageing.
• It arises due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens.
• These days, it is possible to correct to refractive defects with contact lens.

SCIENCE TODY-HUBBLE SPACE TELESCOPE (H.S.T)

• Hubble is the only telescope ever designed to be serviced in space by astronauts.
• Many Hubble observations accurately measure the rate at which the universe is expanding.
• It constrain the value of Hubble’s constant and estimated the age of the universe.
• Hubble’s observation found that black holes are common to the centers of all galaxies.
• The astronomers used the telescope to observe distant supernovae.