physics-class-12-unit-11-chapter-04-what-is-light-i-l-4-5-ces9xz6ui64

### <Success style="font-size:25px"> What Is Light I L-4 </Success>
### <primary style="font-size:24px"> Laser Beam </primary>
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- If a collimated plane wave ( of diameter $2 a$ ) is incident on a lens of focal length $f$, then the wave emerging from the lens will get focused to spot of radius $=\lambda f / a$ which would be of the order of 1 micron.

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<footer style = "font-size: 18px">Laser Beam $\rightarrow$ Laser Beam $\rightarrow$ <span style = "color:blue"> Laser Beam </span> $\rightarrow$ Electromagnetic Spectrum $\rightarrow$ Speed of Light </footer>

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### <primary style="font-size:24px"> Human Eye </primary>
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- Because a laser beam is almost parallel; the lens of the eye will focus them down to a small spot which can cause retinal burns and also can be used for treatment of retinal detachment.

- If the intensity of a laser beam (entering the eye) is $1 \mathrm{mW} / \mathrm{cm}^2$ then the intensity at the retina will be about $100 \mathrm{W} / \mathrm{cm}^2$.

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<footer style = "font-size: 18px">Laser Beam $\rightarrow$ Laser Beam $\rightarrow$ <span style = "color:blue"> Human Eye </span> $\rightarrow$ Human Eye $\rightarrow$ Laser Beam </footer>

### <Success style="font-size:25px"> What Is Light I L-4 </Success>
### <primary style="font-size:24px"> First Book on Optics </primary>
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- In 1015 Alhazen wrote the first book on optics.

- The cover page of a translated edition of Alhazen's OPTICS.

- **Noble Prize winner Abdus Salam said**:

- Alhazen was one of the greatest physicists of all time.

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<footer style = "font-size: 18px">Blue LEDs $\rightarrow$ International Year of Light $\rightarrow$ <span style = "color:blue"> First Book on Optics </span> $\rightarrow$ Essential Light $\rightarrow$ Opticks by Isaac Newton </footer>

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### <primary style="font-size:24px"> Rayliegh Scattering </primary>
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- The visible region of the electromagnetic spectrum

- The sky is blue because the blue component of light gets predominantly (Rayleigh) scattered by the atmosphere.

- Man has also been wanting to know what lights is ???

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<footer style = "font-size: 18px">Rayliegh Scattering $\rightarrow$ Rayliegh Scattering $\rightarrow$ <span style = "color:blue"> Rayliegh Scattering </span> $\rightarrow$ Moon $\rightarrow$ Model of Light </footer>

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### <primary style="font-size:24px"> Christiaan Huygens </primary>
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- Christiaan Huygens, the Dutch astronomer put forward the wave theory of light around 1650.

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<footer style = "font-size: 18px">Model of Light $\rightarrow$ Experiment $\rightarrow$ <span style = "color:blue"> Christiaan Huygens </span> $\rightarrow$ Wave $\rightarrow$ Wave </footer>

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### <primary style="font-size:24px"> Displacement </primary>
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- Displacement at $P$ produced by $S_1$ :

- $y_1=a \cos \left(\omega t-\phi_1\right) ; \phi_1=\frac{2 \pi}{\lambda} S_1 P$

- Displacement at $P$ produced by $S_2$ :

- $y_2=a \cos \left(\omega t-\phi_2\right) ; \phi_2=\frac{2 \pi}{\lambda} S_2 P$

- Resultant displacement at $P: \quad y=y_1+y_2$

- Resultant displacement at $P: \quad y=A \cos \left[\omega t-\frac{\phi_1+\phi_2}{2}\right]$

- $A=2 a \cos \gamma$

- $\gamma=\frac{\phi_1-\phi_2}{2}=\frac{\pi}{\lambda}\left(S_2 P-S_1 P\right) $

- $\Rightarrow I=4 I_0 \cos ^2 \gamma$

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<footer style = "font-size: 18px">Wave $\rightarrow$ Displacement $\rightarrow$ <span style = "color:blue"> Displacement </span> $\rightarrow$ Displacement $\rightarrow$ Thomas Young's Experiment </footer>

### <Success style="font-size:25px"> What Is Light I L-4 </Success>
### <primary style="font-size:24px"> Thomas Young's Experiment </primary>
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- Thomas Young's set up of the interference experiment that he carried out in 1801. Adapted from the Dennis Gabor's Nobel Lecture

- $\lambda = \frac{\beta d}{D}$

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<footer style = "font-size: 18px">Displacement $\rightarrow$ Thomas Young's Experiment $\rightarrow$ <span style = "color:blue"> Thomas Young's Experiment </span> $\rightarrow$ Thomas Young's Experiment $\rightarrow$ Thomas Young's Experiment </footer>

### <Success style="font-size:25px"> What Is Light I L-4 </Success>
### <primary style="font-size:24px"> Thomas Young's Experiment </primary>
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- This time he saw a number of dark lines, regularly spaced; the first clear proof that light added to light can produce darkness. This phenomenon is called interference. Thomas Young had expected it because he believed in the wave theory of light.

- Dennis Gabor in his Nobel Lecture December 11, 1971

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<footer style = "font-size: 18px">Thomas Young's Experiment $\rightarrow$ Thomas Young's Experiment $\rightarrow$ <span style = "color:blue"> Thomas Young's Experiment </span> $\rightarrow$ Thomas Young's Experiment $\rightarrow$ Thank You </footer>

### <Success style="font-size:25px"> What Is Light I L-4 </Success>
### <primary style="font-size:24px"> Thomas Young's Experiment </primary>
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- Light was a wave but how could it propagate through vacuum??

- Animation by Mr Wee Loo Kang Lawrence and Mr Fu-kwun Hwang, author of EJSS 5.0 Francisco Esquembre; used with their kind permission.

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