Virtual Images — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions

Section 1 of 10Meaning of a virtual image

Definition

• Definition an image formed when rays do not meet but appear to meet behind the lens or mirror

Screen

• Screen cannot be projected onto a screen

Orientation

• Orientation always upright

Construction

• Construction trace rays backwards as dashed lines to where they appear to meet

Section 2 of 10Plane mirror images

Properties

• Same size as the object
• Same distance behind the mirror as the object is in front
• Virtual

Formation

• Formation rays reflect with i = r and appear to come from behind the mirror, so the image is where the traced-back rays cross

Section 3 of 10Magnifying glass

Condition

• Condition a converging lens with the object closer than the focal length

Image

• Image virtual, upright, magnified and on the same side of the lens as the object

Use

• Use a single lens used as a magnifying glass

Section 4 of 10Diverging lens

Always virtual

• Always virtual whatever the object distance, the image is virtual

Properties

• Upright
• Diminished
• On the same side of the lens as the object

Construction

• The ray through the centre of the lens does not change direction
• Rays parallel to the principal axis are refracted
• Extend the refracted rays backwards, so they appear to come from a virtual focus

Screen

• Screen no image can be formed on a screen

Section 5 of 10Constructing the image

1

Draw a line from the top of the object through the middle of the lens.

2

Draw a line from the top of the object parallel to the principal axis to the lens.

3

Draw the refracted ray as though it came from the principal focus, continued as a dashed line.

4

The top of the image is where the two lines cross.

Section 6 of 10Comparing the three devices

Device	Image	Size	Orientation
Plane mirror	Virtual	Same size	Upright
Magnifying glass	Virtual	Magnified	Upright
Diverging lens	Virtual	Diminished	Upright

Section 7 of 10Converging and diverging compared

Converging lens

• Converging lens the image can be real (inverted) or virtual (upright)

Diverging lens

• Diverging lens the image is always virtual and upright

Section 8 of 10More from the specification

More from the specification

Magnified virtual images and optical fibres

• Magnified virtual images and optical fibres observing virtual images through ocular lenses magnifies light beams transmitted along bundle optical fibres.

Laws of Reflection

• When a ray of light reflects from a planar boundary, the incident ray, reflected ray,
• and normal to the surface all lie in the same plane,
• and the angle of incidence equals the angle of reflection (θ_i = θ_r).

Total Internal Reflection and Critical Angle

• When light travelling inside an optically dense material strikes a boundary at an angle of incidence greater than the critical angle θ_c, all light is reflected back into the medium.
  • (such as a glass or perspex block)

Calculating the Critical Angle

• The critical angle θ_c is the angle of incidence in the denser medium for which the angle of refraction along the boundary equals 90°, calculated from the refractive index using sin θ_c = 1 / n.

Experimental Tracking with Ray Boxes

• In optics experiments, a thin beam of light from a ray box is directed toward a glass block,
• allowing normal lines, incident angles,
• and refracted ray paths to be marked and measured accurately.

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.

Optical fibre

• Thin transparent strand that guides light by TIR

Optically dense

• Having a high refractive index

Section 9 of 10Key terms

Term	Meaning
Virtual image	Image formed by rays that appear to meet, not projectable on a screen
Magnifying glass	Converging lens used with the object closer than its focal length
Virtual focus	Point from which rays appear to diverge after a diverging lens
Plane mirror	Flat reflecting surface that gives a same-size virtual image
Upright	The same way up as the object

Section 10 of 10Exam technique

1

Saying a virtual image can be caught on a screen — it cannot.

2

Placing the object beyond f for a magnifying glass — the object must be closer than f.

3

Saying a diverging lens can make a real image — its image is always virtual.

4

Drawing the extended rays as solid lines — they are dashed.