Test yourself - Mark scheme Lenses

(a)   (i)

    By using a ruler the final diagram is more likely to be accurate than if drawn freehand. When adding arrows to the lines that represent rays of light make sure that they go in the correct direction and that there are no contradictory arrows. Like the object, the image should always touch the principal axis.

       (ii)   The image is on the same side of the lens as the object.

    This answer is specific. Statements such as ‘in front of the lens’ or ‘behind the lens’ are ambiguous as both statements could refer to either side. Answers such as ‘dashed lines have been used’ do not tell us anything about the image and would not get the mark.

       (iii)  magnification = image height/object height = 4/2 = 2cm 

    It may be difficult to be totally accurate in drawing a ray diagram so a tolerance will be allowed in the magnification value. In this case an answer between 1.8 and 2.2 may be acceptable.

(b)   The image changes from being virtual and upright to being real and inverted. 3

    Two changes have been given; each is worth a mark. The image is still magnified, so this is not a change and therefore should not be mentioned. In order to get this answer you may need to draw another ray diagram. But this time it can just be a quick sketch, as no marks are allocated for producing one.

Test yourself - Lenses

Exam style question about lenses

(a)   The diagram shows an object 3 cm in front of a converging lens of focal length 6 cm.

 

       (i)    Complete a ray diagram to show the position of the image.          (4 marks)

       (ii)   How can you tell from the completed ray diagram that the image formed is virtual?      (1 mark)

       (iii)  Calculate the magnification produced by the lens.

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

(b)   The object is now moved so that it is 8 cm from the lens.

            How does the nature of the image change from when the object was 3 cm in front of the lens?

Convex (Converging) Lenses - Watch before Concave

MyGCSEScience.com video about convex lenses - how to draw the ray diagrams

http://my-gcsescience.com/revision/separate/convex-lenses

If you want to download the notes so that you can practice drawing you can access them here:

Convex lenses notes

Specification reference P3.1.3 Lenses:

• Know that In a convex or converging lens, parallel rays of light are brought to a focus at the principal focus.
• Know the nature of images formed by a converging (convex) lens for an object at different distances
• The construction of ray diagrams to show the formation of images by converging lenses.

Concave (diverging) Lenses

MyGCSEScience.com video about concave lenses - how to draw the ray diagrams

my-gcsescience.com Concave lenses

If you want to download the notes so that you can practice drawing you can access them here:

Concave lens notes

Specification reference P3.1.3 Lenses:

• Know that In a convex or converging lens, parallel rays of light are brought to a focus at the principal focus.
• Know the nature of the image produced by a diverging (concave) lens.
• The construction of ray diagrams to show the formation of images by  diverging lenses.

Test yourself - Mark Scheme The Eye

Mark scheme for questions about The Eye:

(a)   (i)    The light rays change direction.

    Change direction is sufficient for the mark

       (ii)   At the retina

    Remember that the retina is where the image in the eye is formed.

       (iii)  To focus on near objects the ciliary muscle contracts, making the suspensory ligaments slack.  The eye lens becomes fatter and more powerful.

              To focus on distant objects the ciliary muscle relaxes and the suspensory ligaments pull the lens, making it thinner and less powerful.

    The question is really asking two things: first about being able to focus on near objects and second about being able to focus on distant objects. This answer is clear to follow and keeps the explanation for near objects separate from that for distant objects. The key point in each part of the answer is the effect of the ciliary muscle and suspensory ligaments on the shape of the eye lens.

       (iv)  Cornea

    It is easy to forget that the cornea also plays an important part in the refraction and focusing of light in the eye.

(b)   (i)    Short sight. Diverging lenses used to correct short sight have a negative power. 

    There is no mark for simply stating short sight. The key to this question is remembering the type of lenses that have a negative power and then relating this to the correction of short sight.

       (ii)   The eyeball may be longer than it would be for normal sight.

    The other possible cause of short sight that you need to remember is the inability of the eye lens to focus.

       (iii)   f = 1/Power = 1/2.5 = 0.4m

This is clearly set out with substitution shown. Remember that no substitution and a slip with the calculator would mean no marks.

Test Yourself -The Eye Questions

Exam style question about The Eye:

The diagram shows the basic structure of an eye.

(a)   Light entering the eye is refracted by the eye lens

       (i)    What happens to light rays that are refracted?                         (1 mark)

       (ii)   For a person with perfect vision, where in the eye is the light refracted by the eye brought to focus?      (1 mark)

       (iii)  The eye is able to focus on both near and distant objects. Explain how it is able to do this.         (4 marks)

       (iv)  Name the other part of the eye that also refracts light?           (1 mark)(b)   A student has poor eyesight and needs to wear a pair of glasses. Each of the lenses used in the glasses has a power of –2.5 D.

           (i)    Does the student have long sight or short sight?

              Give a reason for your answer.             (1 mark)

(ii)   What eye defect may be causing the student’s poor eyesight?        (1 mark)

(iii)  Calculate the focal length, in metres, of the lenses used in the glasses.

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

The Eye

Another good video from MyGCSEScience.com:

my-gcsescience.com The Eye

Contains information about the function of the parts of the eye, long and short sightedness, and comparing the eye and the camera.

Specification reference P3.1.4 The eye:

• Know the function of the following parts of the eye: retina, lens, cornea, pupil / iris, ciliary muscle, suspensory ligaments.
• understand how the action of the ciliary muscle causes changes in the shape of the lens, which allows the light to be focused at varying distances.
• Be able to explain the cause of long and short sightedness and describe correction of vision using convex and concave lenses to produce an image on the retina.
• Know the range of vision for the eye is between the near point is
  (approximately 25 cm) and the far point (infinity).
• Be able to compare the function and structure of the eye and the camera (including be aware that the film in a camera or the CCDs in a digital camera is the equivalent of the retina in the eye.
• That lasers can be used surgically correct eye problems (Knowledge of how lasers work is not required)