# Evan's Space

## Time interval between two sounds from a gun – direct and reflected sound

Solutions: Option B

You can view the video tutorial here or the written solutions below.

## Light – Reflection – Furthest distance Peter can walk before Joe loses sight of his image

Solutions: Option D

## Demonstration of Reflection, Refraction and TIR using light source with a slit

Reflection of light using a mirror

Refraction of light using various shaped optical mediums

Total Internal Reflection (TIR) using a semi-circular perspex

For Total Internal Reflection (TIR) to occur:
1) angle of incidence i is greater than the critical angle c
2) the light is travelling from an optically denser medium towards a less dense medium

Click on this post for the explanation of TIR

## N98 P1 Q20 Light Reflection – Technique to remember

The diagram shows four lamps in front of a plane mirror. The card prevents the observer at X from seeing the lamps directly, although the image of one lamp can be seen in the mirror.

Which lamp’s image can be seen?

Solution: B

## N2005P1Q24 Sound – Reflection of sound from 2 walls

A man who is standing at a point X between two parallel walls (as shown in the diagram) fires a starting pistol.

He hears the first echo after 0.6s and another one after 0.8s. How long after firing the pistol will he hear the next echo?

Solutions:

View video tutorial to visualise the distance travelled.

Time taken to hear next echo = 0.6 + 0.8 = 1.4s

The distance travelled by sound (left and right) from X is the same during 3rd and 4th echos.

Hence 3rd and 4th echos actually coincide. And time taken is 0.6 + 0.8 = 1.4s.

## Light – Two mirrors in parallel

The diagram below shows a object O placed at the middle of two parallel mirrors. Which of the following is not a possible position of image of O?

Solutions: Option D

Based on the concept of characteristics of mirror image – Distance of object to mirror is equal to Distance of image to mirror.

Object O at the centre of two mirrors, based on the above characteristic, B and C are possible positions of image of O.

Consider A. Treating C as the object now and using the mirror on the left, distance of C to left mirror is equal to distance of A to left mirror.

Hence A is a possible position of image of O.

Consider D. Treating B as object and using the mirror on the right, distance of B to right mirror is not equal to distance of D to right mirror.

Hence D is not a possible position of image of O.

## N2007P1Q20 Light – Mirror Image through Periscope

The diagram shows an object O viewed using two mirrors. A person looks into the mirrors as shown. At which position is the image of O seen.Solutions: Option D

To solve this kind of question, you have to do in stages.

1) Consider the object O and the mirror at the top. The image formed will be equal distance perpendicular to the mirror (follows the 5 characteristics of mirror image). Let the image be O’.

2) Now treat O’ as the new object and consider the mirror at the bottom.  Likewise apply the same principle, the image of O’ is now at position D. Alternatively, a fast and simple way is based on the concept that to our eyes, light seems to travel in straight line. Hence to our eyes, the image of O is directly in front.

## Reflection – Min. length of mirror to see the wall behind

Man is standing at the middle facing the mirror wall. What is the minimum length of mirror on AB that is required to see the whole wall CD?

Use Maths concept on Similar Triangles to solve such questions. If the man is at other positions instead of standing at middle, the min. length of mirror needed can still be solved using Similar triangles.