Magnification & Resolution - A-Level Biology OCR

Key Concepts

• Magnification: Refers to how many times larger an image is compared to the actual object.

  • Example: If an image is 1,000 times larger, its magnification is x 1,000.

• Resolution: The ability to distinguish between two separate points; a measure of the detail an image holds.

  • Example: Two images may have the same magnification, but differing resolutions.

Calculating Magnification

• Formula:
  • [ \text{magnification} = \frac{\text{image size}}{\text{object size}} ]
• Components:
  • Image size: How large the object appears through a microscope.
  • Object size: The real size of the object.
• Unit Consistency: Ensure all measurements are in the same unit, requiring conversion when necessary.

Conversion of Units

• Diagram Provided: Conversion between centimetres, millimetres, micrometres, and nanometres.
• Example conversion: from mm to µm, multiply by 1,000.

Measuring with Microscopes

• Eyepiece Graticule: Used to measure specimen dimensions, scale typically from 1 to 100.
• Calibration:
  • Required for each objective lens due to varying real-world distances represented by graticule divisions.

Calibration with Stage Micrometer

• Stage Micrometer: Glass slide with scale measured in micrometres (µm).
• Steps to Calibrate:
  1. Fix the stage micrometer on the microscope stage.
  2. Align the micrometer scale with the eyepiece graticule.
  3. Count how many graticule divisions fit into one micrometer division.
  4. Calculate the size of each graticule division:
     • [ \text{graticule division} = \frac{\text{size of one micrometer division}}{\text{number of graticule divisions}} ]

Example Calculations

1. Graticule Division Calculation:

   • 40 graticule divisions = 1 micrometer division.
   • Calculation: [ \text{graticule division} = \frac{10}{40} = 0.25 \text{ µm} ]

2. Magnification of Plant Cell:

   • Image Width = 40 mm, Real Width = 20 µm.
   • Convert image size to µm: [ 40 \times 1,000 = 40,000 \text{ µm} ]
   • Magnification: [ \text{magnification} = \frac{40,000}{20} = x 2,000 ]

3. Apparent Size of Sperm Cell:

   • Object Length = 5 µm, Magnification = x 1,500.
   • Rearrange to find image size: [ \text{image size} = 1,500 \times 5 = 7,500 \text{ µm} ]
   • Convert to mm: [ \frac{7,500}{1,000} = 7.5 \text{ mm} ]