Colour filters and polarising filters can be used to enhance photographs - Leaving Cert Physics - Question 7 - 2017

(i) The primary colours absorbed are green and blue.
(ii) The primary colour reflected is red.

When illuminated with green light, the red book would appear (iii) black because it does not reflect any green light.

When illuminated with red light, the red book would appear (iv) red as it reflects red light.

Polarisation refers to the orientation of light waves in a specific direction. It can occur through various methods, such as reflection or absorption through a polarising filter.

This can be demonstrated using two parallel polarising plates and a source of light.

1. Place the two polarising plates over each other.
2. Rotate one plate while observing the light transmitted.
3. Eventually, there will be an angle where no light passes through the plates.

Stress polarisation is commonly used for checking defects in transparent materials, allowing for detection of stress patterns.

The Doppler effect occurs when there is a relative motion between a source of sound and an observer.

1. Draw a diagram showing:
   • A source,
   • An observer, and
   • Non-concentric circular wavefronts emanating from the source.
2. As the source moves towards the observer, the wavelength becomes shorter, resulting in a higher frequency being observed.

To find the speed of the source, we can use the formula:
$f' = f \left( \frac{v}{v - u} \right)$
Given that the observed frequency ( f') is 15% more than the emitted frequency ( f) and the speed of sound in air ( v) is 340 m/s, we can calculate it as follows:
Let the emitted frequency be f.
Then,
f' = 1.15f.
Using the formula:
$1.15f = f \left( \frac{340}{340 - u} \right)$
Simplifying, we find:
$1.15 = \frac{340}{340 - u}$
Cross-multiplying gives:
$1.15(340 - u) = 340$
Solving for u yields:
u = 44.3 ext{ m s}^{-1}.