Extended writing questions - lenses, refraction etc...

Some ideas for possible questions...

1) Describe an experiment you could do, including the measurements you would need to take, to determine the refractive index of a transparent block.

2) Compare the similarities and differences between images formed by a magnifying glass and a diverging lens.

3) Describe the factors which affect the focal length of a lens and explain how the lenses in modern glasses can be manufactured to be very thin.

4) Describe an experiment that could be carried out to determine the critical angle of a glass block.

5) Explain how total internal reflection occurs. Why is total internal not possible if light is travelling in air towards a glass surface?

6) Explain how an endoscope works and it's uses in medicine.

Answers to follow shortly.

Test yourself markscheme - Refraction, refractive index and total internal reflection

Mark scheme for exam questions - How did you do???

(a)   (i)    Critical angle

       (ii)   The light is totally internally reflected. 

       It would be insufficient to simply say that ‘the light is reflected’ or even 

      ‘reflected back into the block’.

       (iii)   refractive index =  1/ sin c 

                                        = 1/ sin 42

                                        = 1/ 0.669 = 1.5

       Alternatively the equation refractive index = sin i/sin r could have been used.      

       However, if you use this equation you must imagine the light to be travelling from

       air into glass and not the other way around. So take the angle of incidence as 

       either 31° or 49° with the associated angle of refraction either 20° or 30°.

(b)

• An endoscope has two bundles of optical fibres. 
• Light that is directed into one bundle travels from one end to the other by being totally internally reflected many times. 
• The light is then reflected back through the second bundle of optical fibres.
• The image is viewed through a lens, 
• or if a miniature camera is attached to the optical fibres the image can be displayed on a computer screen.

    It is perfectly okay to answer a ‘prose’ type question using bullet points. However, if 

    you do, make sure each bullet point is a complete sentence.

Test yourself - Refraction, Refractive Index and Total Internal Reflection

Exam style question about applications of refraction and total internal reflection

A student investigates the refraction of light as it passes out from a glass block into the air.

  

Each time the student changes the angle of incidence (i) in the glass block he measures the angle of refraction (r) in the air. The student’s results are given in the table.

Angle of incidence (i)	Angle of refraction (r)
20°	31°
30°	49°
42°	90°
50°	no refracted ray
70°	no refracted ray

(a)   (i)    What name is given to the angle of incidence that produces an angle of refraction equal to 90°?                                      (1 mark)

       (ii)   What happens to the light once the angle of incidence exceeds 42°? (1 mark)

       (iii)  Use data from the table to calculate the refractive index of the glass block.

              Write down the equation you need to use and show how you work out your answer.      (2 marks)

(b)   A doctor may use an endoscope to look directly inside a patient’s body. Explain how an endoscope works.  (4 marks)

Refractive Index

MyGCSE Science Video about Refractive Index

http://www.my-gcsescience.com

Specification reference:
P3.1.3 Lenses

• Refraction is the change of direction of light as it passes from one medium to another.
• A lens forms an image by refracting light
• refractive index =  sin i /sin r, where i is the angle of incidence and r is the angle of refraction

P3.1.4 The Eye

• The focal length of a lens is determined by:
•  the refractive index of the material from which the lens is made, and
•  the curvature of the two surfaces of the lens
• For a given focal length, the greater the refractive index, the flatter the lens. This means that the lens can be manufactured thinner.