Light: Laws of Reflection & Human Vision

In this power point presentation, we discuss about the light, the laws of reflection, how we are able to see things, about human eye and how human vision works. We also talk about application of reflection of light by discussing about periscope and kaleidoscope.

1. LIGHT CLASS 8 th PREPARED BY: BHAVYA VASHISHT

2. Introduction Light is form of energy Light is needed to see things around us We see various things around us with eyes but eyes alone can’t see any object. We can’t see objects in dark We need source of light to make objects visible It is light which makes things visible to us. Light enables us to see things from which it comes or from which it is reflected

3. LUMINOUS & NON LUMINOUS OBJECTS Luminous objects : Objects which emit their own light They are source of light Very small in number Example: Stars, Lighted electric bulb, fire etc. Non- Luminous objects : Objects which don’t emit their own light Can’t make their own light Most objects around us are non luminous. Can be seen only when light coming from luminous objects fall on them Non luminous objects are also called Illuminated Objects Example: Planets, satellites, book, chair, etc.

4. REFLECTION OF LIGHT Process of sending back light rays which fall on surface of an object is called reflection of light. Studied by using plane mirror Plane mirror reflects almost all light falling on it Plane mirror changes direction of light falling on it RAY BOX EXPERIMENT O (Goes in) (Goes out) Point of Incidence Plane A B M M’

5. We use Ray Box to produce thin beam of light It has narrow slit in front of box When light bulb is switched on, very thin beam of light comes out of narrow slit This narrow beam of light used to study reflection of light from plane mirror Thin beam of light produced by ray box is visible on white sheet of paper, so its path can be traced by using pencil. Though thin beam of light is made up of several rays of light but for simplicity & convenience, a thin beam of light is considered to be a ray of light

6. TERMINOLOGY Incident Ray : *Ray of light which falls on the mirror surface *Tells us direction in which light from source falls on mirror *Always go towards the mirror Point of Incidence : *The point at which incident ray strikes mirror *It tells us where exactly light falls on mirror surface Reflected Ray : *Ray of light which is sent back by mirror *Tells us direction in which light goes after reflection from mirror * Always go away from the mirror There can be only 1 reflected ray for given incident ray falling on plane mirror. Same ray of light is called incident ray before it strikes the mirror & becomes reflected ray after it rebounds from mirror

7. TERMINOLOGY Normal: *Line drawn at right angles to mirror at point of incidence *It is just an imaginary line *Lies exactly in between incident and reflected ray Angle of Incidence: *Angle between incident ray and normal *Represented by “ i ” Angle of Reflection: *Angle between reflected ray and normal *Represented by “r”

8. LAWS OF reflection of Light FIRST LAW OF REFLECTION The incident ray, reflected Ray & normal all lie on same plane SECOND LAW OF REFLECTION The angle of incidence is always equal to angle of reflection ∠ i = ∠r

9. M M’ N EXAMPLE 1

10. EXAMPLE 2 EXAMPLE 3

11. FORMATION OF IMAGE IN PLANE MIRROR Consider the figure given above. Here object AB of height ‘h’ is placed at a distance ‘u’ from the mirror. When the rays of light from this object fall on the mirror, the image of the object is formed inside the mirror. Consider the incident rays that fall on the mirror: AP, AO and BO. These incident rays are reflected back from the mirror as PA, OC and OB. The rays PA and OC are diverging rays hence we can extend them behind the mirror to find the point of intersection that is A’. Similarly, the other reflected rays are extended backwards and virtual image A’B’ of height ‘h’ of the object AB is formed on the mirror.

12. CHARACTERISTICS OF IMAGE FORMED BY PLANE MIRROR Is the same size as that of the object Left-right inverted (Lateral Inversion) Erect and virtual Formed behind the mirror at the same distance as that of the object placed in front of the mirror The object and image are equidistant from the mirror.

13. Angle between 2 plane mirrors No. of images formed 180° 1 120° 2 90° 3 60° 5 45° 7 0° Infinite

14. 180 ° 120 ° 9 0 ° 6 0 ° 45 ° 0 °

15. HUMAN EYE The front part of the eye is covered with a transparent spherical membrane known as the cornea. Light enters the eye by cornea.  Space present just behind the cornea contains a fluid known as aqueous humour . Just behind the cornea is a muscular diaphragm, which is dark coloured and is known as the iris which has a small circular opening in the middle called the pupil. The black colour of the pupil is due to no light being reflected from it.  The iris is responsible for controlling the amount of light entering the eye by adjusting the size of the pupil. The lens of our eyes is a convex lens made of a transparent jelly-like proteinaceous material. The eye lens is hard in the middle and becomes soft towards the outer edges.  The ciliary muscles hold the eye lens in its position. The ciliary muscles are responsible for changing the curvature and focal length of the eye lens. The inner back surface of the eyeball is the retina. It is a semi-transparent membrane that is light sensitive and behaves as the screen of a camera. The light-sensitive receptors present in the retina are rods and cone cells.  The space between the retina and eye lens is filled with vitreous humour .

16. Functions of Iris and Pupil Iris automatically adjusts the size of pupil according to intensity of light received by the eye from the surroundings. If amount of light is high, iris contracts the pupil & hence reduces amount of light entering the eye. If the amount of light around us is small, iris expands the pupil so that more light can enter the eye Adjustment of size of pupil takes some time

17. RODS, CONES, BLIND SPOT Rods are the rod shaped cells present in the retina of an eye which are sensitive to dim light Rods don’t provide information about the color of objects Cones are the cone shaped cells present in retina of an eye which are sensitive to bright light. Cones also cause sensation of color of objects in our eyes. Blind spot is small area of retina insensitive to light where optic nerve leaves the eye. When the image is formed at blind spot in the eye, it can’t be seen by the eye.

18. PERSISTENCE OF VISION The image of an object seen by our eyes persists on retina for about 1/16 th of a second even after the object has disappeared from our view. The ability of an eye to continue to see the image of an object for a very short duration even after the object has disappeared from view is called persistence of vision. If still pictures of a moving object are flashed on our eyes at a rate faster than 16 pictures per second, then eyes perceive this object as moving.

19. RANGE OF VISION Far point : Farthest point from the eye at which an object can be seen clearly. It is at infinity Near point: Nearest point up to which the eye can see an object clearly without any strain. I t is about 25 cm Normal eye can see distant objects as well as nearby objects by focusing images of distant objects as well as nearby objects on its retina by changing thickness (converging power) of its lens Thin eye lens has less converging power whereas thick eye lens has greater converging power on the rays of light coming from an object When eye is looking at distant object, ciliary muscles gets relaxed due to which eye lens is thin (less convex). Thin eye lens has smaller converging power which is sufficient to converge parallel rays of light coming from distant object to form its image on retina. When eye is looking at nearby object, ciliary muscles gets stretched due to which eye lens is thick (more convex). Thick eye lens has greater converging power which is required to converge diverging rays of light coming from nearby object to form its image on retina

20. DEFECTS OF THE EYE Defects in the eye happen due to many reasons. Due to growing age, the vision also decreases, and when the focal length alters, the vision also alters. C ataract is the common defect seen in the eye. Cataracts cause partial or sometimes complete loss of vision, when not treated properly. When the crystalline lens at old age becomes milky and cloudy it is known as a cataract. When a person undergoes cataract surgery, the vision can be restored. When the eye loses its ability to adjust its focal length, problems appear like a person cannot see the image properly (blurring of vision), unable to view nearby objects or far away objects. If not taken timely care, eyes might completely lose the power of accommodation. Myopia is commonly known as near-sightedness . In this condition, the person can see the objects nearby, but cannot see distant objects clearly. Faraway objects appear blurry and a person will not be comfortable in seeing them.  Myopia  condition takes place when the shape of eyes leads the light rays to bend in a wrong way, focusing images in front of the retina rather than focusing on the retina. Correction  : When a concave lens of suitable power is used, it assists in focusing the image onto the retina.

21. Hypermetropia or Far-Sightedness Hypermetropia is commonly known as far-sightedness. In this condition, the person can see objects at distance but cannot see nearby objects clearly. Usually, the person with this disorder squints to see nearby objects.  Hypermetropia  is caused when the light rays from a closeby object are focussed at a point behind the retina. Correction  : Using spectacles with a converging lens (Convex) imparts additional focussing power and thus helps in forming the image on the retina. DEFECTS OF THE EYE

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