A student determines the focal length of a converging lens using two experimental methods.
The student sets the distance UUU between an illuminated object and the lens. She moves the screen until a sharp image is seen on the screen.
A scale diagram of this setup is drawn to a scale of 1/4th1/4^{\text{th}}1/4th actual size. On this diagram, she measures:
Calculate the actual distance UUU and the actual distance VVV in the experiment.
Calculate the focal length f1f_1f1 of the lens using the equation:
f1=UVU+V f_1 = \frac{UV}{U+V} f1=U+VUVBriefly describe a technique to obtain an image on the screen that is as sharp as possible in this experiment.
The student keeps the screen in the same position. She moves the lens closer to the screen until another sharp image is seen.
On a scale diagram of this second setup (also drawn to 1/4th1/4^{\text{th}}1/4th actual size), she measures:
Calculate the actual distance VVV between the lens and the screen for this second setup.
The heights of the object and the image on the screen are measured to be:
Calculate the magnification MMM using the equation:
M=hIhO M = \frac{h_I}{h_O} M=hOhICalculate a second value f2f_2f2 for the focal length of the lens using the equation:
f2=VM+1 f_2 = \frac{V}{M+1} f2=M+1VState one precaution the student must take to ensure that the measurements of UUU and VVV in this experiment are as reliable as possible.
Explain why measuring the height of the image hIh_IhI in Method 2 is likely to produce a less accurate value for the focal length than the distance measurements in Method 1.
Suggest one improvement to make the measurement of the image height in Method 2 more accurate.
216 exam-style questions on CIE IGCSE Physics 3.2 Light, covering 3.2.1a Reflection of light, 3.2.1b Images in a plane mirror, 3.2.2 Refraction of light, 3.2.2a Refraction of light, 3.2.2b Total internal reflection, 3.2.2c Refractive index and critical angle, 3.2.2d Optical fibres, 3.2.3a Converging and diverging lenses, 3.2.3b Ray diagrams and the nature of images, 3.2.3c Virtual images and the magnifying glass, 3.2.3d Correcting long-sightedness and short-sightedness, and 3.2.4 Dispersion of light. Each one has a worked solution and a mark scheme showing where the marks go.