Ray Optics And Optical Instruments Class 12 Notes Physics Chapter 9 - CBSE
Chapter : 9
What are Ray Optics and Optical Instruments ?
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| Topic | Formula | Symbol Representation | Important Points |
| Apparent Depth | Apparent depth = (real dept/µ) | µ=refractive index =(sin i/sin r)(Snell's law) i = incident angle of ray r = angle of refraction |
When we look into a pool of water, the bottom of the pool will appear to be raised due to refraction of light. |
| Critical Angle | c = sin-1(1/µ) or μ = (1/sin c) | μ = refractive index of denser medium w.r.t. rarer medium c = critical angle which depends on colour of light |
Phenomenon of refraction of light into denser medium from the boundary of denser medium with rarer medium known as total internal reflections i > ic Some of the important applications of total internal reflection: (a) Diamond brilliance (b) Optical fiber (c) Mirage |
| Lens Maker’s Formula | (1/f)=(µ-1) ( (1/R1)-(1/R2)) | μ = refractive index of material of lens w.r.t. medium in which lens is placed. R1, R2 = radii of curvature of the two surfaces of the lens. |
This formula is valid for all types of lenses. |
| Thin Lens Formula | (1/v)-(1/u) =( 1/f) | u = distance of the object from the optical centre of the lens v = distance of image from the optical centre of the lens f = focal length of the lens |
f is positive for converging or convex lens and negative for diverging or concave lens. |
| Linear Magnification | m=(I/O)-(V/U) | v = size of image u = size of object |
m is positive for virtual image and m is negative for real image. |
| Topic | Formula | Symbol Representation | Important Points |
| Power of a Lens | P = (1/f) | f = focal length in metres Unit of power of lens = dioptre = 1D = 1m-1 |
• Ability of lens to converge or diverge a beam of light. • P is positive for a convex lens and negative for a concave lens. • When focal length of a lens is in cm, then P =(100/f(in cm))dioptre |
| Combination of then lenses | •(1/F1)+(1/F2)+(1/F) • P = P1 + P2 • m = m1 × m2 |
F1, F2 = Focal length of two thin lenses places coaxially F = focal length of combination P = Power of lens with proper sign m1, m2 = magnification |
Combination of lens used for: • Increase the magnification of image • Make the final image erract w.r.t. object • Reduce certain observations |
| Angle of deviation | δ = i1 + i2 – A | i1 = angle of incidence i2 = angle of emergence A = r1 + r2 = angle of prism |
The difference between the sum of incident angles and the sum of emergent angles is known as the angle of deviation. |
| Prism Formula | μ = sin[(A+δm)/2]/sin(A/2) (Snell’s law) |
A = angle of prism δm= angle of minimum deviation i = angle of incidence δm = 2 i – A when i1 = i2 and r1 = r2 |
• For thin prism, Sin [A+δm]/2→(A+δm/2) and Sin (A/2)→(A/2) • δ = (μ–1)A this is the formula of angle of maximum deviation. |
| Angular Dispersion | θ = δV – δR = (µV – μR) A |
δV, δR = Deviation of violet and red light μV, μR = Refractive index for violet and Red colour. |
The difference in the angle of deviation between two extreme colours is known as angle of dispersion or angular dispersion for the two colours. |
| Magnifying Power of Simple Microsope | • Image, at the least distance of vision m=1+(D/f) • Image at infinity m=(D/f) |
m = magnification D = least distance vision |
A simple microscope or a simple magnifying glass is a converging lens of small focal length. |
| Magnifying Power of Compound Microscope | • Image, at infinity m=(-L/v0(D/fe)) • Image, at the least distance of vision m =(-Vo/Uo(1+(D/fe)) |
uo = distance of object from the objective lens vo=L=distance of image from the objective lens (length of microscope tube) fe = focal length of the eye lens D = least distace of distinct vision. |
• Compound microscope is the combination of two convex lenses one compounding the effect of the other, for much larger magnification. • The objective have smaller aperture and smaller focal length than the eye-piece. |
| Topic | Formula | Symbol Representation | Important Points |
| Mirror Formula | (1/u)+(1/v)=(1/f) | u = distance of object from the pole of the mirror v = distance of image f = (R/2) |
• Mirror formula is same for both concave and convex mirrors equation remain uneffected whether the image is real or virtual. |
| Linear Magnification | • m =(size of image (h2)/size of object (h1)) • m =(f/f-v) |
f = focal length of magnifying lens. | In case of concave mirror, when image is real, m = negative. When image is virtual, m is positive. |
| Use of spherical mirror | • As reflector in street lamps, search light. • Driving mirror • Telescopes, solar cookers |
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| Optical fiber | • Based on total internal reflection. • Use for data transmission using light pulses. • Made of plastic or glass. |
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| Magnifying Power Astronomical Telescope (Refracting Type) | • When the final image is formed at infinity (Normal Adjustment), m=(fo/fe) • When the final image is formed at least distance of distinct vision, m =(-fo/fe)(1+(fe/D)) |
fo = Focal length of objective lens fe = Focal length of the eye-piece D = least distance of distinct vision (i.e. 25 cm) |
• Astronomic telescope (Refracting type) is consist of two converging lense the objective lens has large focal length and large aperture than the eye-piece. |
| Magnifying Power of Reflecting Type Telescope | In normal adjustment m =(-fo/fe)=((R/2)/fe) |
R = radius of curvature of concave mirror | • It is improvement over refracting type telescope. Here, the objective lens is replaced by a concave parabolic mirror. |
