These notes for CBSE Class 12 Physics cover Ray Optics and Optical Instruments. They detail the laws of reflection and the Cartesian sign convention for spherical mirrors, along with the mirror formula and magnification. The notes explain refraction, Snell's law, refractive index, and the principle of reversibility. Key phenomena like apparent depth shift and Total Internal Reflection (TIR) are discussed, including its conditions and applications such as optical fibers and the brilliance of diamonds. The text also covers refraction through spherical surfaces, the lens maker's formula, and combinations of lenses. Furthermore, it explains refraction through prisms, dispersion of light, Cauchy's formula, and scattering of light, explaining phenomena like the blue sky and reddish sunsets. Finally, the resolving power of microscopes and telescopes is defined. These notes are ideal for quick revision and understanding of core concepts for exams.
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1. The incident ray, normal and reflected ray all lie in the same plane. 2. The angle of incidence is always equal to angle of reflection.
1. All the distance measurements are done from pole of the mirror and height
measurements are from the principal axis.
2. The distances measured in the direction of incident ray are positive and in
opposite direction of incident ray are negative.
3. The heights measured upwards from principal axis are positive and downwards
negative.
•The relation between focal length and radius of curvature of spherical mirror
is f = R/2
•Mirror formula where u is the object distance, v is the image
distance and f is the focal length.
•Linear magnification is ratio of the size of the image to the size of the
object.Magnification If m >1 then the image is magnified.
If m<1 then the image is diminished. If m=1 then the image is of the same size of
the object. If m is positive then the image is virtual and erect. If m is negative then
the image is real and inverted.
•The phenomenon of bending of light at the interface of two media when it
enters from one medium to another is called refraction of light.
1. The incident ray, normal and refracted ray all lie in the same plane. 2. The ratio of sine of angle of incidence and sine of angle of refraction is constant for a given pair of media.
The ratio of sine of angle of incidence and sine of
angle of refraction is the refractive index of refracting medium with respect to the
incident medium.
•The ratio of speed of light in free space to that in medium is called absolute
refractive index of that medium.
•If the absolute refractive index medium 1 is greater than that of medium 2
then medium 1 is said to be optically denser medium and medium 2 is said to be
optically rarer medium.
•The ratio of absolute refractive index of one medium to that of another is
called relative refractive index of both the media.
The relative refractive index of medium 2 w.r.t.
medium 1 is reciprocal to the relative refractive index of medium 1 w.r.t. medium 2. •An object under water ( any medium ) appears to be raised due to refraction
when observed inclined and Shift in the position (apparent) of object is X = t { 1 – 1/n)
where t is the actual depth of the medium.
•The Phenomenon of reflection of light completely in to the denser medium
without refraction in to rarer medium is called Total Internal Reflection.
1. The light must propagate from optically denser medium to optically rarer
medium.
2. The angle of incidence in the denser medium must be greater than critical angle
of incidence for the given pair of media.
•When a ray of light travels from denser to rarer medium and if the angle of
incidence is greater than critical angle, the ray of light is reflected back to the
denser medium. This phenomenon is called Total internal reflection. D R n n
SinC ?
Brilliance of diamond, totally reflecting prisms, Mirage, Looming, Optical Fibre.
When light falls on a convex
refracting surface, it bends and the relation between U, V and R is given by
•Lens maker’s formula or thin lens formula is given by
•For Convex Lens R1 +ve ;R2 –ve Concave lens R1-ve; R2 +ve.
•When two lenses are kept in contact the equivalent focal length is given by • •Combination of lenses help to
1)Increase the magnification of the image
2) Increase the sharpness of the final image by reducing the defects of images
formed by a singe lens.
3) Make the image erect w.r.t the object
4) Increase the field of view
•The lens formula is given by
•When light passes through a
glass prism it undergoes refraction.
The expression for refractive index is As the angle of incidence increases, the angle of
deviation decreases, reaches a minimum value and then increases. This minimum
value is called angle of minimum deviation “Dm”.
•The phenomenon of splitting up of polychromatic light in to its constituent
colours is called Dispersion of light.
•Cause of dispersion: According to Cauchy’s formula, the refractive index
(n) of a material depends upon wavelength (?) and is given by:
•? =a + b/?2 + c/?4, where a, b, c are constants.
•Scattering of light takes place when size of the particle is very small when
compared to the wavelength of light. Intensity of scattered light is
•The following properties or phenomena can be explained by scattering.
1. Sky is blue.
2. Sun is reddish at the time of sunrise and sunset
3. Infra-red photography used in foggy days.
4. Orange colour of black Box
5. Yellow light used in vehicles on foggy days.
6. Red light used in signals.
Resolving power of microscope is the ability of the microscope to show as separate images of two point objects lying close to each other. Resolving power=2nsin ?/? nsin ? is called numerical aperture of the microscope. Resolving power of telescope
is its ability to show distinctly the images of two distant objects lying close by.
Resolving power =D/1.22? D is the diameter of the objective
The laws of reflection state that the incident ray, normal, and reflected ray all lie in the same plane, and the angle of incidence is equal to the angle of reflection.
The mirror formula relates object distance (u), image distance (v), and focal length (f) of a spherical mirror. It is typically expressed as 1/v + 1/u = 1/f.
Snell's law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media, and this constant is the refractive index of the refracting medium.
TIR occurs when light travels from an optically denser to a rarer medium, and the angle of incidence in the denser medium is greater than the critical angle.
The lens maker's formula relates the focal length of a lens to its refractive index and the radii of curvature of its surfaces.
Dispersion of light occurs because the refractive index of a material depends on the wavelength of light, as described by Cauchy's formula.
The sky appears blue due to the scattering of sunlight by atmospheric particles. Shorter wavelengths (blue light) are scattered more effectively than longer wavelengths.
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