Light, Reflection, Refraction - Cbse Class 10 Physics

Gurukul 24x7 – We Teach You To Excel Light, Reflection and Refraction 1. Light travels in a straight line.Can be seen from the sharp shadow of the opeque object casted by source of light 2. Light is both a matter made up of photons and also consist electromagnetic waves.Different waves have different frequencies and colors.So it exhibits properties of both matter and waves. 3. The light incident on an object can be reflected, absorbed and transmitted through the object. 4. The color of the object is color of the light reflected by the object. 5. A dull/unpolished object absorbs most of the light falling on it and hence reflects less percent of incident light. Example- black board is black since it absorbs most of the incident light 6. Transparent object transmits most of the light falling on it.Example- glass 7. A bright object reflects most of the incident light.Example- polished silver mirror 8. Reflection of light is the phenomenon of sending the incident light rays back. 9. Surface of silver can be easily scratched and it soon becomes rough.So it is made by depositing a thin layer of silver on one side of plane glass sheet.The silver layer is protected by red paint.Light rays travel through the glass and reflects from silver side. 10. Angle of incidence and of angle of reflection are measured with the normal to the mirror at the point of incidence. 11. First law of reflection: Incident ray, normal to the mirror and reflected ray all lie in same plane. 12. Second law of Reflection: Angle of incidence=angle of reflection N

R

I

Q O

P NO =normal to PQ IO=incident ray OR=reflected ray IO,NO and OR lie in one plane in which normal is made I.e. plane NOQ and NOP ION=Angle of incidence=NOR (angle of reflection) *PQ can be surface of plane or curved mirror. So if the angle x is made with the plane of the mirror, angle of incidence=angle made with the normal=90-x 13. We use silver for making mirrors because it is a good reflector of light. 14. How is an image formed? ● A number of light rays from the object will be incident on the mirror. ● The light rays incident on the mirror will be reflected according to laws of reflection. ● The point/points where the reflected rays meet or appear to come from will be the point

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where the image is formed. When the reflected rays actually meet, we get a real image that can be captured or observed or obtained on a screen.For eg: images on cinema screen. When reflected rays do not actually meet but appear to come/diverge from a point, we get a virtual image that can't be captured on a screen.Example : Convex mirror, plane mirror

eye

Image Location (from where reflected rays appear to diverge from---Virtual Image)

eye In order to see the image of an object in a mirror, you must sight at the image; when you sight at the image, light will come to your eye along that line of sight.

1. 2. 3. 4.

Reflection by plane mirror Image size =size of object Distance of object in front of the mirror=Distance of (virtual) image behind the mirror. Lateral inversion of the image formed. Image formed is always erect

Spherical Mirrors – Concave and Convex Mirrors –obtained by cutting a part of the hollow sphere as shown.So center of the sphere from which the curved or spherical mirror is obtained is called the center of curvature of the spherical mirror.Center of curvature doesn't lie on the curved mirror. ● The pole lies at the center of the surface of curved surface. ● Greater the radius of curvature of the sphere, greater the focul length (R=2F) ● Shorter focul length implies larger converging/diverging power image distance h2 -image distance from pole −v = = = ● Magnification= (be careful with object distance h1 object distance from pole u the sign of distances) ● Mirror Formula for Spherical mirrors:

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1 1 1 1 1  =  = (be object distance from pole image distance from pole u v focul length f  careful with the sign of distances) Thickness of the lens is inversely proportional to its focul length .If the lens is thick then it means it has been made from a sphere with smaller radius so it has smaller focul length.Similarly if the lens is thin then it has been made from a sphere of larger radius, so it is has been made from a large sphere.You need to imagine this in your mind or just draw 2 circles of different sizes on a piece of paper.

Rules to remember 1. Incident rays from the object that are parallel to principal axis, pass through the focal point after being reflected from the mirror. 2. Incident rays from the object that pass through the focus, become parralel after being reflected from the mirror. 3. Incident ray passing through the center of curvature, are reflected back through the center of curvature. (Rays passing through the center of curvature are normal to the lens surface because then the ray is along the radius and tangent along the lens boundary will be normal to the radius) Concave mirror – (Converging mirror) 1. 2. 3. 4.

Real focus on left side of the pole (Focul length is -ve) Real and inverted image is formed except for the case when object is between F and P Virtual and erect image is produced when object is between F and P Produces a powerful parallel beam of light when object is placed on focus.Because of this, it is used in torches,vehicle head lights etc. 5. Used in -solar cookers ,In solar furnaces -because they converge the solar light to its focus 6. by dentist, in shaving mirrors, in make up mirros because they give virtual enlarged image when object I brought close to it (between F and P) Convex mirror – (Diverging mirror)

Gurukul 24x7 – We Teach You To Excel 1. Virtual focus on right side of the pole (focul length is +ve) 2. Always gives virtual, erect and diminished image of the object 3. If object moves away from the pole of convex mirror, image becomes smaller in size and moves away from the pole. 4. Convex mirror has a wide field or range of view since it always produces diminished images. It is therefore used in rear-view mirrors or side-mirrors of vehicles. REFRACTION

Figure 1 1. When particles of light interact with the electrons of the atoms of the medium then particles of the medium absorb energy from light particles called photon.The electrons remit some energy in the form of light or electromagnetic wave.Since the interaction between photons and electrons of the medium is different,the speed of light varies in different medium. 2. Light travels with different speed in different medium depending upon its optical density.Speed of light is more in an optically rarer medium than in optically denser medium. 3. Refraction occurs at the boundry of the two mediums 4. Due to refraction light bends towards or away from the normal when it travels from one medium to another.It moves towards the normal when travels from optically rarer to denser medium and away from the normal when it travels from optically denser medium to optically rarer medium. Snell's Law if light travels from medium 1 to medium 2 , then the relation between angle of incidence on the boundary of medium 1 and angle of reftaction in medoum 2 is give by sin i refractive index of medium 2 n2 = = =refractive index of medium 2 with respect to sin r refractive index of medium 1 n1 medium 1 Refractive index is a measure of the optical density of the medium.Higher the refractive index, higher the optical density.

Gurukul 24x7 – We Teach You To Excel *Refractive index of air and vaccum is 1.So when medium 1 is air or vaccum, sini/sinr=absolute refractive index of medium 2. So for figure 1,

sin Q1 n glass = ---Equation 1 sin Q2 n air

If ray refracts at B with angle Q3, then snell's law when light travels into air from point B will be A Q1 O

Q2

B Q sin Q2 n air = ---------Equation 2 sin Q3 n glass Multiplying equation 1 and 2,

sin Q1 =1 sin Q3

P

=> Q3=Q1

Q3 is called emergent angle (e) . Emergent angle=Incident angle only when two faces where refraction occurs are parallel to each other.Otherwise angle COB !=angle OBD=Q2 5. Refraction depends upon the angle of incidence and the nature of the medium. 6. Refractive index of the medium depends upon the frequency of incident light (or its color). 7. Perpedicular distance between incident and emergent ray is called lateral displacement. Speed of the light is inversely proportional to the refractive index of the medium. => Speed of light in medium 2 n1 = speed of light in medium 1 n2 8. Speed of light in vaccum = 3 x 10 8

Convex Lens and Concave Lens

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A double convex lens or convex lens (biconvex lens) as well as concave lens can be thought of as originally being a part of the sphere but both forned in different manner as shown.

Refraction through convex lens:

*Refraction occurs twice at the boundries between are and glass.. *Normal to incident and refracted rays is drawn from the center of the spheres of which the lenses are a part of. *Optical center is the center of the convex of concave lens and is different from the center of the sphere from which the lenses are made. Magnification formula for lenses: m=h2/h1= v/u Lens formula:

Gurukul 24x7 – We Teach You To Excel 1/v -1/u=1/f Concave Mirror

Concave Lens

Focus in front of the mirror (on left side) because parallel incident rays meet at a point on the left after reflection. So focul length is negative.

Focus on left side (or same side as the object) of the lens because parallel rays incident on the concave lens appear to diverge from point on the left after refraction.Focul length is negative

Since parallel rays actually meet after being reflected so focus point is real.

Since parallel rays appear to diverge after refraction so focus point is virtual.

Forms both virtual or real image depending on position of the object.

Always forms virtual image.

Convex Mirror

Convex Lens

Focus behind the mirror because parallel incident rays appear to come from that point after reflection from convex mirrror.Focul length Is +ve

Focus on right side (or other side of the object) of the lens because parallel rays incident on the convex meet at a point on the right side (or other side of the object) after refraction.Focul length is +ve

Since parallel rays appear to come/diverge from Since parallel rays actually meet after being focus point after being reflected so focus point is being refracted to focus point is real. virtual. Forms only virtual image.

Mirror

Forms both real or virtual image depending on the position of the object.

Lens

Real image is formed in front of the mirror (on left or same side as the object) because on that side only converging rays actually meet while virtual image is formed behind the mirror (on right or opposite side of the object ) since rays cannot pass through the mirror so appear to diverge from a virtual image.

Real image is formed on right (or opposite side of the object) since rays pass through lens converge to meet on the other side while Virtual image is formed on the left (same side as the object) since rays passing through the lens are diverged and cannot meet so appear to come from virtual image.

Mirror Formula : 1/v+1/u=1/f

Lens Formula: 1/v-1/u=1/f

Magnification Relation with Object & Image Distance: h2 / h1 =-v / u

Magnification Relation with Object & Image Distance: h2 / h1 =v / u