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3.2.8Waves

Real Images — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 11 parts Lesson 22 of 35 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

3.2.8 Real Images — IGCSE PHYSICS notes: Explain how a converging lens forms real images and describe their position, size and uses.Converging lens — bends rays from each point of the object so they meet at a corresponding point on the far sideOutside f — when the object is beyon…

  • Rays from the top
  • Rays from the bottom
  • Shorter focal length
  • Optical centre
  • Focal point
  • Magnification

Part 1 of 9How a real image forms

Converging lens
  • Converging lens bends rays from each point of the object so they meet at a corresponding point on the far side
Outside f
  • Outside f when the object is beyond the focal length the rays from the tip cross at one point and form a real image
Screen
  • Screen light passes through the image point, so a screen placed there shows a sharp picture
Blurring
  • Blurring a screen too close or too far gives a blurred picture

Part 2 of 9Three construction rules

Rule 1
  • Rule 1 rays through the optical centre pass through the lens undeviated
Rule 2
  • Rule 2 rays parallel to the principal axis are refracted through the focal point f
Rule 3
  • Rule 3 rays through the focal point f emerge parallel to the principal axis

Part 3 of 9Image for different object positions

Object positionImage formsNatureOrientationSize
Beyond 2fBetween f and 2fRealInvertedDiminished
At 2fAt 2fRealInvertedSame size
Between f and 2fBeyond 2fRealInvertedMagnified

Part 4 of 9Why real images are inverted

Rays from the top
  • Rays from the top cross the axis and end up below it
Rays from the bottom
  • Rays from the bottom end up above the axis
Result
  • Result the image is upside down and left and right are swapped

Part 5 of 9Drawing a diminished real image

1
Draw and label the principal axis and the lens.
2
Mark and label the focal points on both sides.
3
Draw and label the object further than 2f from the lens.
4
Draw a ray from the top of the object through the optical centre.
5
Draw a second ray parallel to the principal axis, then through the focal point on the far side.
6
Draw and label the image where the rays meet.
  • Marks
    • one ray through the centre, a second ray through a labelled focal point, and object and image both drawn and labelled

Part 6 of 9Finding a real image experimentally

1
Set up a lit object, a converging lens on a holder and a white screen on a metre ruler.
2
Find f by focusing a distant window on the screen, since the lens–screen distance is about f.
3
Place the object at a distance u greater than f.
4
Move the screen until the image is sharp and measure the image distance v and the heights.
5
Repeat for u beyond 2f, near 2f and between f and 2f.
Sharpness
  • Sharpness move the screen back and forth and take the middle of the sharp range
Distances
  • Distances measure from the centre of the lens

Part 7 of 9Cameras, projectors and the eye

Camera
  • Camera the object is beyond 2f, giving a small, inverted real image on a sensor or film
Projector
  • Projector the slide is between f and 2f, giving a large, inverted real image on a screen
  • The slide is loaded upside down so the screen image is upright
Eye
  • Eye the cornea and lens form a real, inverted, diminished image on the retina

Part 8 of 9Magnification

Equation
  • Equation magnification = image height ÷ object height, which also equals v ÷ u
Shorter focal length
  • Shorter focal length gives a larger magnification at the same screen distance
Near f
  • Near f as the object approaches f from outside the image grows rapidly but becomes dimmer

Part 9 of 9More from the specification

More from the specification
Projected real images and optical fibres
  • Projected real images and optical fibres focusing light onto end faces of coherent optical fibres transmits real images across medical scopes.
Real vs Virtual Images
  • A real image is formed by actual converging light rays and can be projected onto a screen,
  • whereas a virtual image is formed by diverging rays that only appear to intersect behind an optical boundary.
Law of Reflection
  • Light reflecting from a plane mirror maintains strict equality between the angle of incidence and the angle of reflection, both measured from the perpendicular normal.
Total Internal Reflection
  • Occurs when light traveling inside a medium of higher refractive index strikes the boundary at an angle of incidence exceeding the critical angle, undergoing 100% internal reflection.
Diverging and Converging Ray Systems
  • Convex lenses converge incident rays to form real, inverted images for objects beyond the principal focus,
  • whereas concave lenses produce diverging rays yielding upright, virtual images.
Real vs Virtual Images
  • A real image is formed by actual converging light rays and can be projected onto a screen,
  • whereas a virtual image is formed by diverging rays that only appear to intersect behind an optical boundary.
Law of Reflection
  • Light reflecting from a plane mirror maintains strict equality between the angle of incidence and the angle of reflection, both measured from the perpendicular normal.
Total Internal Reflection
  • Occurs when light traveling inside a medium of higher refractive index strikes the boundary at an angle of incidence exceeding the critical angle, undergoing 100% internal reflection.
Diverging and Converging Ray Systems
  • Convex lenses converge incident rays to form real, inverted images for objects beyond the principal focus,
  • whereas concave lenses produce diverging rays yielding upright, virtual images.
Optically dense
  • Having a high refractive index
Reflection
  • Light bouncing off a surface.
Boundary
  • Surface between two media at which waves are partially reflected
Normal
  • Line at 90° to a surface.
Refractive index
  • Ratio of the speed of light in a vacuum to its speed in a material
Critical angle
  • Incline angle at which sliding just begins.
Virtual image
  • Image formed where rays only appear to meet, not projectable on a screen
Optical fibre
  • Thin transparent strand that guides light by TIR
Plane mirror
  • Flat mirror.

Key terms

TermMeaning
Real imageImage formed where rays converge and meet, projectable on a screen
Optical centreCentre of a lens through which rays pass undeviated
Focal pointPoint where parallel rays converge after a converging lens
MagnificationImage height divided by object height
DiminishedSmaller than the object

Avoid these mistakes

1
Saying real images are upright — they are always inverted.
2
Placing the object inside f and expecting a screen image — that gives a virtual image.
3
Measuring distances from the lens holder edge — measure from the centre of the lens.
4
Drawing the second ray without a labelled focal point — the ray must pass through f.

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